Amount of Substance (Edexcel International A Level (IAL) Chemistry): Flashcards
Exam code: YCH11
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Define: concentration
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Define: concentration
Concentration is the amount of a substance dissolved in a given volume of solution.
Molar concentration has units of mol dm-3. Mass concentration has units of g dm-3.
What is the equation for molar concentration?
Molar concentration (mol dm-3) = number of moles / volume of solution (dm3).
Equivalently: moles = concentration x volume.
Mass concentration = mass of solute (g) divided by ..........
Mass concentration = mass of solute (g) divided by volume of solution (dm3).
This gives units of g dm-3.
True or False?
To convert cm3 to dm3, divide by 1000.
True.
1 dm3 = 1000 cm3, so dividing cm3 by 1000 gives dm3. This conversion is essential in concentration calculations.
What is parts per million (ppm) as a concentration unit?
Parts per million (ppm) is a unit for expressing very low concentrations. 1 ppm equals 1 mg of solute dissolved in 1 dm3 (1 kg) of solution.
How is the concentration of an atmospheric gas calculated in ppm?
Atmospheric gas concentration (ppm) = (volume of gas / volume of air) x 1,000,000.
Both volumes must be in the same units.
A solution contains 6.34 g of NaCl (Mr = 58.5) in 250 cm3. The molar concentration is ..........
A solution contains 6.34 g of NaCl (Mr = 58.5) in 250 cm3. The molar concentration is 0.434 mol dm-3.
Moles = 6.34 / 58.5 = 0.1084 mol. Concentration = 0.1084 / 0.250 = 0.434 mol dm-3.
True or False?
The gas involved does not affect the ppm calculation for atmospheric gas concentration.
True.
The formula (volume of gas / volume of air) x 1,000,000 applies regardless of which gas is being measured. Only the volumes matter.
Why must volumes be in the same units when calculating atmospheric gas concentration in ppm?
The formula divides volume of gas by volume of air. If the units differ, the ratio, and therefore the ppm value, will be incorrect.
Convert one volume to match the other before dividing.
What is a reacting mass calculation?
A reacting mass calculation uses a balanced chemical equation and molar masses to find the mass of a reactant or product involved in a reaction.
What are the steps to calculate the mass of a product from the mass of a reactant?
Write the balanced equation.
Find the molar mass of each substance.
Convert the given mass to moles.
Use the molar ratio to find moles of the target substance.
Multiply moles by molar mass to get mass.
In a reacting mass calculation, moles of a substance = mass divided by ..........
In a reacting mass calculation, moles of a substance = mass divided by molar mass.
Molar mass has units of g mol-1.
True or False?
In a reacting mass calculation, it is only valid to work in grams.
False.
As long as the same mass unit is used consistently for all substances, the reacting masses remain proportional to the balanced equation.
How is a balanced equation determined from the reacting masses of substances?
Convert each mass to moles using molar mass. Divide all mole values by the smallest to get a whole-number molar ratio. Apply these ratios as the coefficients in the balanced equation.
7.5 g of Mg (Mr = 24.3) reacts with oxygen in a 1:1 ratio to form MgO (Mr = 40.3). The mass of MgO formed is ..........
7.5 g of Mg (Mr = 24.3) reacts with oxygen in a 1:1 ratio to form MgO (Mr = 40.3). The mass of MgO formed is 12.4 g.
Moles of Mg = 7.5 / 24.3 = 0.309 mol. Mass of MgO = 0.309 x 40.3 = 12.4 g.
True or False?
The molar ratio in a balanced equation always equals the ratio of masses of the substances.
False.
The molar ratio gives the ratio of moles, not masses. To compare masses, the moles must be multiplied by each substance's molar mass.
Why is the balanced equation essential in a reacting mass calculation?
The balanced equation provides the molar ratio between reactants and products. Without the correct ratio, the number of moles of the target substance cannot be determined.
How do you deduce the molar ratio of two substances from a balanced equation?
Read the stoichiometric coefficients in front of each formula. The ratio of these coefficients gives the molar ratio of the substances in the reaction.
What is the molar gas volume?
Molar gas volume is the volume occupied by one mole of any gas under specified conditions. At r.t.p. it is 24.0 dm3. At s.t.p. it is 22.4 dm3.
What are the conditions for room temperature and pressure (r.t.p.)?
Room temperature and pressure (r.t.p.) is defined as 293 K (20 °C) and 101.3 kPa. The molar gas volume at r.t.p. is 24.0 dm3.
Number of moles of gas = volume of gas (dm3) divided by ..........
Number of moles of gas = volume of gas (dm3) divided by molar gas volume (dm3).
True or False?
At standard temperature and pressure (s.t.p.), the molar gas volume of any gas is 24.0 dm3.
False.
At s.t.p. (273 K / 0 °C and 101.3 kPa) the molar gas volume is 22.4 dm3. The value of 24.0 dm3 applies at r.t.p. (293 K).
Define ideal gas
An ideal gas is a theoretical gas whose molecules have negligible volume, no intermolecular forces, undergo elastic collisions, and whose temperature is proportional to average kinetic energy.
State the ideal gas equation and define each variable.
The ideal gas equation is PV = nRT, where:
P = pressure (Pa), V = volume (m3), n = moles of gas (mol), R = 8.31 J K-1 mol-1, T = temperature (K).
In the ideal gas equation PV = nRT, pressure must be in pascals (Pa) and volume must be in ..........
In the ideal gas equation PV = nRT, pressure must be in pascals (Pa) and volume must be in m3.
Convert: 1 kPa = 1000 Pa; 1 dm3 = 0.001 m3.
True or False?
Real gases behave exactly like ideal gases under all conditions.
False.
Real gases only approximate ideal behaviour. They deviate because molecules have finite volume and experience intermolecular forces, particularly at high pressure and low temperature.
How do you convert a Celsius temperature to kelvin for use in the ideal gas equation?
Add 273 to the Celsius temperature to get the temperature in kelvin (K).
For example, 21 °C = 21 + 273 = 294 K.
Why can the volumes of gases at equal temperatures and pressures be compared directly without converting to moles?
All gases occupy the same molar gas volume under the same conditions of temperature and pressure. The volume ratio equals the mole ratio, so gas volumes can be used directly in stoichiometric calculations.
Define percentage yield
Percentage yield is the actual yield expressed as a percentage of the theoretical yield:
percentage yield = (actual yield / theoretical yield) x 100
Give three reasons why the percentage yield of a reaction may be less than 100%.
Other reactions occur simultaneously.
The reaction does not go to completion.
Products are lost during separation and purification.
Define atom economy
Atom economy is the molecular mass of the desired product expressed as a percentage of the total molecular mass of all reactants:
atom economy = (mass of desired product / sum of masses of all reactants) x 100
True or False?
Addition reactions always have an atom economy of 100%.
True.
In an addition reaction, all atoms from the reactants are incorporated into the single product, so no atoms are wasted. The RN states: "In addition reactions, the atom economy will always be 100%."
Atom economy = (molecular mass of desired product / ..........) x 100
Atom economy = (molecular mass of desired product / sum of molecular masses of ALL reactants) x 100.
Why might a calculated percentage yield exceed 100%, and what errors could cause this?
A percentage yield over 100% indicates an experimental error. Possible causes include: the product crystals were still wet when weighed, impurities are present in the product, or the mass of the container was included in the product mass.
True or False?
A reaction with only one product always has an atom economy of 100%.
True.
If there is only one product, all atoms from the reactants end up in that product, giving 100% atom economy. The RN confirms: "Whenever there is only one product, the atom economy will always be 100%."
How is atom economy calculated using masses rather than Mr values?
Atom economy (by mass) = (mass of desired product formed / total mass of all reactants) x 100.
This gives the same result as using Mr values because mass is proportional to the number of moles.
In the reaction Fe2O3 + 3CO → 2Fe + 3CO2, the atom economy for iron is 2 x 55.8 divided by .......... x 100
In the reaction Fe2O3 + 3CO → 2Fe + 3CO2, the atom economy for iron is 2 x 55.8 divided by (159.6 + 3 x 28.0) x 100.
This gives 111.6 / 243.6 x 100 = 45.8%.
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