Define a mole in chemistry.

A mole is the SI unit of amount of substance. One mole of any substance contains 6.02 x 10 23 particles (atoms, molecules, or ions). This number is known as the Avogadro constant .

One mole of any substance contains ______ particles. This number is called the ______ constant.

One mole of any substance contains 6.02 x 10 23 particles. This number is called the Avogadro constant.

True or False? One mole of carbon atoms and one mole of water molecules contain the same number of particles.

True. One mole of any substance always contains 6.02 x 10 23 particles, regardless of what the substance is. The Avogadro constant applies to atoms, molecules, and ions alike.

What is the molar mass of a substance, and how is it related to relative atomic or molecular mass?

The molar mass is the mass of one mole of a substance, in g/mol. For an element, it equals the relative atomic mass in grams. For a compound, it equals the relative formula mass or relative molecular mass in grams.

True or False? To find the number of moles of a gas, you divide the volume by the Avogadro constant.

False. To find moles of a gas at RTP, divide the volume in dm³ by 24 . The Avogadro constant (6.02 x 10 23 ) is used to find the number of particles from moles, not to find moles from volume.

A sample contains 1 mole of hydrogen gas (H 2 ). How many hydrogen atoms does it contain?

1 mole of H 2 contains 6.02 x 10 23 molecules. Since each H 2 molecule has 2 hydrogen atoms, the total number of atoms = 2 x 6.02 x 10 23 = 1.204 x 10 24 atoms.

The number of moles of a substance is calculated by dividing its ______ by its ______ .

The number of moles is calculated by dividing its mass (in grams) by its molar mass (g/mol) .

Calculate the number of moles in 36 g of water (H 2 O). ( M r of H 2 O = 18)

Moles = mass / molar mass = 36 / 18 = 2 mol

True or False? To find the mass of 0.5 mol of CaCO 3 ( M r = 100), you calculate 100 / 0.5.

False. Mass = moles x molar mass = 0.5 x 100 = 50 g . Dividing the molar mass by moles gives the wrong answer. Use: mass = moles x M r .

Why is it important to show all working in stoichiometry calculations?

Showing working allows error carried forward (ECF) marks to be awarded. If the final answer is wrong but the method is clearly shown, marks can still be gained for correct steps. Without working, no ECF credit is possible.

To calculate the mass of a substance from the number of moles, use: mass = ______ x ______ .

To calculate the mass of a substance from the number of moles, use: mass = moles x molar mass

Calculate the mass of 0.25 mol of magnesium (Mg). ( A r of Mg = 24)

Mass = moles x molar mass = 0.25 x 24 = 6 g

At room temperature and pressure (RTP), one mole of any gas occupies ______ dm 3 or ______ cm 3 .

At room temperature and pressure (RTP), one mole of any gas occupies 24 dm 3 or 24 000 cm 3 .

Calculate the number of moles of CO 2 in 6 dm 3 of the gas at RTP.

Moles = volume / 24 = 6 / 24 = 0.25 mol

True or False? To find the moles of a gas from a volume given in cm 3 , you divide the volume by 24.

False. If the volume is in cm 3 , divide by 24 000 (not 24) to find moles at RTP. Alternatively, first convert cm 3 to dm 3 by dividing by 1000, then divide by 24. Dividing cm 3 by 24 gives an answer 1000 times too large.

State Avogadro's Law and explain why it simplifies reacting gas volume calculations.

Avogadro's Law states that equal volumes of gases contain equal numbers of moles at the same temperature and pressure. This means the ratio of coefficients in a balanced equation equals the ratio of gas volumes, so reacting gas volumes can be calculated directly from molar ratios.

In the reaction C 3 H 8 (g) + 5O 2 (g) → 3CO 2 (g) + 4H 2 O (g), if 200 cm 3 of propane reacts completely: The volume of O 2 required is ______ cm 3 The volume of CO 2 produced is ______ cm 3

In the reaction C 3 H 8 (g) + 5O 2 (g) → 3CO 2 (g) + 4H 2 O (g), if 200 cm 3 of propane reacts completely: The volume of O 2 required is 1000 cm 3 The volume of CO 2 produced is 600 cm 3

True or False? At the same temperature and pressure, 2 mol of oxygen gas and 2 mol of nitrogen gas occupy the same volume.

True. By Avogadro's Law , equal numbers of moles of any gas occupy the same volume at the same temperature and pressure. Both samples would occupy 2 x 24 = 48 dm 3 at RTP.

What are the two common units for expressing the concentration of a solution?

Concentration can be expressed in g/dm 3 (grams per cubic decimetre) or mol/dm 3 (moles per cubic decimetre). Note that 1 dm 3 = 1000 cm³.

To convert a volume from cm 3 to dm 3 , you ______ by 1000. To convert from dm 3 to cm 3 , you ______ by 1000.

To convert a volume from cm 3 to dm 3 , you divide by 1000. To convert from dm 3 to cm 3 , you multiply by 1000.

True or False? You can calculate concentration in mol/dm 3 using the formula concentration = moles / volume, even if the volume is given in cm 3 .

False. The formula requires the volume in dm 3 , not cm 3 . Always divide the volume in cm 3 by 1000 to convert to dm 3 before calculating. Using cm 3 directly gives an answer 1000 times too small.

How many moles of HCl are present in 25 cm 3 of a 0.2 mol/dm 3 solution?

Convert: 25 / 1000 = 0.025 dm 3 . Moles = concentration x volume = 0.2 x 0.025 = 0.005 mol

The formula for calculating concentration in mol/dm 3 is: concentration = ______ / ______ (in dm 3 ).

The formula for calculating concentration in mol/dm 3 is: concentration = moles / volume (in dm 3 )

Calculate the concentration in mol/dm 3 of a solution containing 0.01 mol of NaOH in 250 cm 3 of water.

Convert: 250 / 1000 = 0.25 dm³. Concentration = 0.01 / 0.25 = 0.04 mol/dm³

Define limiting reactant .

The limiting reactant is the reactant that is completely used up first in a reaction. It limits the amount of product that can be formed. The other reactant is said to be in excess .

In a reacting masses calculation, the molar ratio between substances is found from the ______ equation. Moles are calculated using: moles = ______ / molar mass.

In a reacting masses calculation, the molar ratio between substances is found from the balanced equation. Moles are calculated using: moles = mass / molar mass.

True or False? To identify the limiting reactant, you can directly compare the masses of the two reactants given in the question.

False. Masses cannot be directly compared. You must first calculate the moles of each reactant and compare these using the molar ratio from the balanced equation. Direct mass comparison ignores molar ratios and gives the wrong answer.

In the reaction 2Mg + O 2 → 2MgO, calculate the mass of MgO produced from 4.8 g of Mg. ( A r : Mg = 24; M r : MgO = 40)

Moles of Mg = 4.8 / 24 = 0.2 mol. Ratio Mg:MgO = 1:1, so moles of MgO = 0.2 mol. Mass of MgO = 0.2 x 40 = 8.0 g

True or False? In a reacting masses calculation, you should always use the moles of the limiting reactant to find the mass of product.

True. The limiting reactant determines the maximum yield of product. Using the moles of the reactant in excess would give a mass of product that is impossible to obtain.

Why does the amount of product formed depend on the limiting reactant and not the reactant in excess?

Once the limiting reactant is completely used up, the reaction stops regardless of how much of the other reactant remains. The maximum amount of product is set by the moles of the limiting reactant and the molar ratio in the balanced equation.

In a titration calculation, the moles of a solution are calculated using: moles = ______ x ______ (volume in dm 3 ).

In a titration calculation, the moles of a solution are calculated using: moles = concentration x volume (in dm 3 )

True or False? A burette reading of exactly zero should be recorded as "0" in a results table.

False. Burette readings must always be recorded to two decimal places . A reading of zero must be written as 0.00 , and a reading of 15 cm 3 as 15.00 . This indicates the precision of the measurement.

25.0 cm 3 of 0.100 mol/dm 3 NaOH is neutralised by 20.0 cm 3 of HCl. Calculate the concentration of the HCl. (NaOH:HCl ratio = 1:1)

Moles NaOH = 0.100 x 0.0250 = 0.00250 mol. Moles HCl = 0.00250 mol (1:1 ratio). Concentration HCl = 0.00250 / 0.0200 = 0.125 mol/dm 3

True or False? The mean titre is calculated by averaging all titration results, including rough titrations.

False. The mean titre uses only consistent (concordant) results — typically those within 0.10 cm 3 of each other. Rough titrations and outliers are excluded from the mean.

How can you check that you have not inverted the mole ratio in a titration calculation?

Consider the relative volumes used: a larger volume means the solution is more dilute (lower concentration). If your calculated concentration does not match this expectation relative to the other solution, the ratio has been inverted.

Titration calculations should typically be expressed to ______ decimal places, matching the precision of ______ readings.

Titration calculations should typically be expressed to 2 decimal places, matching the precision of burette readings.

To find the empirical formula from mass data, divide the mass of each element by its ______ , then find the simplest whole number ______ .

To find the empirical formula from mass data, divide the mass of each element by its relative atomic mass ( A r ), then find the simplest whole number ratio .

True or False? When calculating an empirical formula, the mass of hydrogen should be divided by 2, since hydrogen exists as H 2 .

False. Always divide by the relative atomic mass ( A r ) of the element, not its molecular mass. For hydrogen, use A r = 1 (not 2). Empirical formula calculations use individual atoms, not molecules.

A compound contains 6 g of carbon and 2 g of hydrogen. Find its empirical formula. ( A r : C = 12, H = 1)

Moles: C = 6/12 = 0.5; H = 2/1 = 2. Ratio C:H = 0.5:2 = 1:4 . Empirical formula = CH 4

True or False? If calculating an empirical formula gives the ratio C:H:O = 1:2:1.5, the empirical formula is C 1 H 2 O 1.5 .

False. The empirical formula must have whole number ratios. Multiply all values by 2 to give C:H:O = 2:4:3. The empirical formula is therefore C 2 H 4 O 3 .

To find the molecular formula from the empirical formula, divide the M r of the ______ formula by the M r of the ______ formula, then multiply each subscript by this value.

To find the molecular formula from the empirical formula, divide the M r of the molecular formula by the M r of the empirical formula, then multiply each subscript by this value.

The empirical formula of a compound is CH 2 O and its M r is 60. Find the molecular formula. ( A r : C = 12, H = 1, O = 16)

M r of CH 2 O = 12 + 2 + 16 = 30. Multiplier = 60 / 30 = 2. Molecular formula = C 2 H 4 O 2

Percentage yield = ( ______ yield / ______ yield) x 100

Percentage yield = ( actual yield / theoretical yield) x 100

True or False? A percentage yield greater than 100% is possible if a reaction is very efficient.

False. Percentage yield can never exceed 100%. A value above 100% means the calculation is wrong — most often the actual and theoretical yields have been divided the wrong way around.

Give three reasons why the percentage yield of a reaction is always less than 100% in practice.

Reasons why the percentage yield is typically less than 100%: The reaction may be reversible Reactants or products may be lost during transfer between containers Products may be lost during separation or purification Side reactions may consume some reactants Reactants may remain on equipment .

Percentage by mass of an element = (total mass of element in compound / ______ of compound) x ______

% by mass = (total mass of element / relative formula mass of compound) x 100

A student obtained 1.6 g of copper(II) sulfate. The theoretical yield was 2.0 g. Calculate the percentage yield.

% yield = (actual / theoretical) x 100 = (1.6 / 2.0) x 100 = 80%

True or False? Percentage purity is calculated by dividing the mass of the pure substance by the total mass of the sample, then multiplying by 100.

True. Percentage purity = (mass of pure substance / total mass of substance) x 100. A perfectly pure sample has a percentage purity of 100% .

Calculate the percentage by mass of iron in iron(III) oxide, Fe 2 O 3 . ( A r : Fe = 56, O = 16)

M r of Fe 2 O 3 = (2 x 56) + (3 x 16) = 160. Mass of Fe = 2 x 56 = 112. % by mass = (112 / 160) x 100 = 70%

O LevelChemistryCambridge (CIE)Flashcards3. Stoichiometry3.3 The Mole & the Avogadro Constant

3.3 The Mole & the Avogadro Constant (Cambridge (CIE) O Level Chemistry): Flashcards

Exam code: 5070

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FrontThe Mole
Define a mole in chemistry.

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Cards in this collection (49)

Define a mole in chemistry.
A mole is the SI unit of amount of substance. One mole of any substance contains 6.02 x 10²³ particles (atoms, molecules, or ions). This number is known as the Avogadro constant.
One mole of any substance contains ______ particles. This number is called the ______ constant.
One mole of any substance contains 6.02 x 10²³ particles. This number is called the Avogadro constant.
True or False?
One mole of carbon atoms and one mole of water molecules contain the same number of particles.
True.
One mole of any substance always contains 6.02 x 10²³ particles, regardless of what the substance is. The Avogadro constant applies to atoms, molecules, and ions alike.
What is the molar mass of a substance, and how is it related to relative atomic or molecular mass?
The molar mass is the mass of one mole of a substance, in g/mol. For an element, it equals the relative atomic mass in grams. For a compound, it equals the relative formula mass or relative molecular mass in grams.
True or False?
To find the number of moles of a gas, you divide the volume by the Avogadro constant.
False.
To find moles of a gas at RTP, divide the volume in dm³ by 24. The Avogadro constant (6.02 x 10²³) is used to find the number of particles from moles, not to find moles from volume.
A sample contains 1 mole of hydrogen gas (H₂). How many hydrogen atoms does it contain?
1 mole of H₂ contains 6.02 x 10²³ molecules. Since each H₂ molecule has 2 hydrogen atoms, the total number of atoms = 2 x 6.02 x 10²³ = 1.204 x 10²⁴ atoms.
The number of moles of a substance is calculated by dividing its ______ by its ______.
The number of moles is calculated by dividing its mass (in grams) by its molar mass (g/mol).
Calculate the number of moles in 36 g of water (H₂O). (M_r of H₂O = 18)
Moles = mass / molar mass = 36 / 18 = 2 mol
True or False?
To find the mass of 0.5 mol of CaCO₃ (M_r = 100), you calculate 100 / 0.5.
False.
Mass = moles x molar mass = 0.5 x 100 = 50 g. Dividing the molar mass by moles gives the wrong answer. Use: mass = moles x M_r.
Why is it important to show all working in stoichiometry calculations?
Showing working allows error carried forward (ECF) marks to be awarded. If the final answer is wrong but the method is clearly shown, marks can still be gained for correct steps. Without working, no ECF credit is possible.
To calculate the mass of a substance from the number of moles, use:
mass = ______ x ______.
To calculate the mass of a substance from the number of moles, use:
mass = moles x molar mass
Calculate the mass of 0.25 mol of magnesium (Mg). (A_r of Mg = 24)
Mass = moles x molar mass = 0.25 x 24 = 6 g
At room temperature and pressure (RTP), one mole of any gas occupies ______ dm³ or ______ cm³.
At room temperature and pressure (RTP), one mole of any gas occupies 24 dm³ or 24 000 cm³.
Calculate the number of moles of CO₂ in 6 dm³ of the gas at RTP.
Moles = volume / 24 = 6 / 24 = 0.25 mol
True or False?
To find the moles of a gas from a volume given in cm³, you divide the volume by 24.
False.
If the volume is in cm³, divide by 24 000 (not 24) to find moles at RTP. Alternatively, first convert cm³ to dm³ by dividing by 1000, then divide by 24. Dividing cm³ by 24 gives an answer 1000 times too large.
State Avogadro's Law and explain why it simplifies reacting gas volume calculations.
Avogadro's Law states that equal volumes of gases contain equal numbers of moles at the same temperature and pressure. This means the ratio of coefficients in a balanced equation equals the ratio of gas volumes, so reacting gas volumes can be calculated directly from molar ratios.
In the reaction C₃H₈ (g) + 5O₂ (g) → 3CO₂ (g) + 4H₂O (g), if 200 cm³ of propane reacts completely:
- The volume of O₂ required is ______ cm³
- The volume of CO₂ produced is ______ cm³
In the reaction C₃H₈ (g) + 5O₂ (g) → 3CO₂ (g) + 4H₂O (g), if 200 cm³ of propane reacts completely:
- The volume of O₂ required is 1000 cm³
- The volume of CO₂ produced is 600 cm³
True or False?
At the same temperature and pressure, 2 mol of oxygen gas and 2 mol of nitrogen gas occupy the same volume.
True.
By Avogadro's Law, equal numbers of moles of any gas occupy the same volume at the same temperature and pressure. Both samples would occupy 2 x 24 = 48 dm³ at RTP.
What are the two common units for expressing the concentration of a solution?
Concentration can be expressed in g/dm³ (grams per cubic decimetre) or mol/dm³ (moles per cubic decimetre). Note that 1 dm³ = 1000 cm³.
To convert a volume from cm³ to dm³, you ______ by 1000.
To convert from dm³ to cm³, you ______ by 1000.
To convert a volume from cm³ to dm³, you divide by 1000.
To convert from dm³ to cm³, you multiply by 1000.
True or False?
You can calculate concentration in mol/dm³ using the formula concentration = moles / volume, even if the volume is given in cm³.
False.
The formula requires the volume in dm³, not cm³. Always divide the volume in cm³ by 1000 to convert to dm³ before calculating. Using cm³ directly gives an answer 1000 times too small.
How many moles of HCl are present in 25 cm³ of a 0.2 mol/dm³ solution?
Convert: 25 / 1000 = 0.025 dm³. Moles = concentration x volume = 0.2 x 0.025 = 0.005 mol
The formula for calculating concentration in mol/dm³ is:
concentration = ______ / ______ (in dm³).
The formula for calculating concentration in mol/dm³ is:
concentration = moles / volume (in dm³)
Calculate the concentration in mol/dm³ of a solution containing 0.01 mol of NaOH in 250 cm³ of water.
Convert: 250 / 1000 = 0.25 dm³. Concentration = 0.01 / 0.25 = 0.04 mol/dm³
Define limiting reactant.
The limiting reactant is the reactant that is completely used up first in a reaction. It limits the amount of product that can be formed. The other reactant is said to be in excess.
In a reacting masses calculation, the molar ratio between substances is found from the ______ equation. Moles are calculated using:
moles = ______ / molar mass.
In a reacting masses calculation, the molar ratio between substances is found from the balanced equation. Moles are calculated using:
moles = mass / molar mass.
True or False?
To identify the limiting reactant, you can directly compare the masses of the two reactants given in the question.
False.
Masses cannot be directly compared. You must first calculate the moles of each reactant and compare these using the molar ratio from the balanced equation. Direct mass comparison ignores molar ratios and gives the wrong answer.
In the reaction 2Mg + O₂ → 2MgO, calculate the mass of MgO produced from 4.8 g of Mg. (A_r: Mg = 24; M_r: MgO = 40)
Moles of Mg = 4.8 / 24 = 0.2 mol.
Ratio Mg:MgO = 1:1, so moles of MgO = 0.2 mol.
Mass of MgO = 0.2 x 40 = 8.0 g
True or False?
In a reacting masses calculation, you should always use the moles of the limiting reactant to find the mass of product.
True.
The limiting reactant determines the maximum yield of product. Using the moles of the reactant in excess would give a mass of product that is impossible to obtain.
Why does the amount of product formed depend on the limiting reactant and not the reactant in excess?
Once the limiting reactant is completely used up, the reaction stops regardless of how much of the other reactant remains. The maximum amount of product is set by the moles of the limiting reactant and the molar ratio in the balanced equation.
In a titration calculation, the moles of a solution are calculated using:
moles = ______ x ______ (volume in dm³).
In a titration calculation, the moles of a solution are calculated using:
moles = concentration x volume (in dm³)
True or False?
A burette reading of exactly zero should be recorded as "0" in a results table.
False.
Burette readings must always be recorded to two decimal places. A reading of zero must be written as 0.00, and a reading of 15 cm³ as 15.00. This indicates the precision of the measurement.
25.0 cm³ of 0.100 mol/dm³ NaOH is neutralised by 20.0 cm³ of HCl. Calculate the concentration of the HCl. (NaOH:HCl ratio = 1:1)
Moles NaOH = 0.100 x 0.0250 = 0.00250 mol.
Moles HCl = 0.00250 mol (1:1 ratio).
Concentration HCl = 0.00250 / 0.0200 = 0.125 mol/dm³
True or False?
The mean titre is calculated by averaging all titration results, including rough titrations.
False.
The mean titre uses only consistent (concordant) results — typically those within 0.10 cm³ of each other. Rough titrations and outliers are excluded from the mean.
How can you check that you have not inverted the mole ratio in a titration calculation?
Consider the relative volumes used: a larger volume means the solution is more dilute (lower concentration). If your calculated concentration does not match this expectation relative to the other solution, the ratio has been inverted.
Titration calculations should typically be expressed to ______ decimal places, matching the precision of ______ readings.
Titration calculations should typically be expressed to 2 decimal places, matching the precision of burette readings.
To find the empirical formula from mass data, divide the mass of each element by its ______, then find the simplest whole number ______.
To find the empirical formula from mass data, divide the mass of each element by its relative atomic mass (A_r), then find the simplest whole number ratio.
True or False?
When calculating an empirical formula, the mass of hydrogen should be divided by 2, since hydrogen exists as H₂.
False.
Always divide by the relative atomic mass (A_r) of the element, not its molecular mass. For hydrogen, use A_r = 1 (not 2). Empirical formula calculations use individual atoms, not molecules.
A compound contains 6 g of carbon and 2 g of hydrogen. Find its empirical formula. (A_r: C = 12, H = 1)
Moles: C = 6/12 = 0.5; H = 2/1 = 2.
Ratio C:H = 0.5:2 = 1:4.
Empirical formula = CH₄
True or False?
If calculating an empirical formula gives the ratio C:H:O = 1:2:1.5, the empirical formula is C₁H₂O_1.5.
False.
The empirical formula must have whole number ratios. Multiply all values by 2 to give C:H:O = 2:4:3. The empirical formula is therefore C₂H₄O₃.
To find the molecular formula from the empirical formula, divide the M_r of the ______ formula by the M_r of the ______ formula, then multiply each subscript by this value.
To find the molecular formula from the empirical formula, divide the M_r of the molecular formula by the M_r of the empirical formula, then multiply each subscript by this value.
The empirical formula of a compound is CH₂O and its M_r is 60. Find the molecular formula. (A_r: C = 12, H = 1, O = 16)
M_r of CH₂O = 12 + 2 + 16 = 30. Multiplier = 60 / 30 = 2. Molecular formula = C₂H₄O₂
Percentage yield = (______ yield / ______ yield) x 100
Percentage yield = (actual yield / theoretical yield) x 100
True or False?
A percentage yield greater than 100% is possible if a reaction is very efficient.
False.
Percentage yield can never exceed 100%. A value above 100% means the calculation is wrong — most often the actual and theoretical yields have been divided the wrong way around.
Give three reasons why the percentage yield of a reaction is always less than 100% in practice.
Reasons why the percentage yield is typically less than 100%:
- The reaction may be reversible
- Reactants or products may be lost during transfer between containers
- Products may be lost during separation or purification
- Side reactions may consume some reactants
- Reactants may remain on equipment.
Percentage by mass of an element = (total mass of element in compound / ______ of compound) x ______
% by mass = (total mass of element / relative formula mass of compound) x 100
A student obtained 1.6 g of copper(II) sulfate. The theoretical yield was 2.0 g. Calculate the percentage yield.
% yield = (actual / theoretical) x 100 = (1.6 / 2.0) x 100 = 80%
True or False?
Percentage purity is calculated by dividing the mass of the pure substance by the total mass of the sample, then multiplying by 100.
True.
Percentage purity = (mass of pure substance / total mass of substance) x 100.
A perfectly pure sample has a percentage purity of 100%.
Calculate the percentage by mass of iron in iron(III) oxide, Fe₂O₃. (A_r: Fe = 56, O = 16)
M_r of Fe₂O₃ = (2 x 56) + (3 x 16) = 160.
Mass of Fe = 2 x 56 = 112.
% by mass = (112 / 160) x 100 = 70%

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