Define empirical formula

An empirical formula is the simplest whole-number ratio of atoms of each element in a compound or molecule.

The relative formula mass ( M r ) is calculated by .......... the A r values of all atoms in the formula.

The relative formula mass ( M r ) is calculated by adding the A r values of all atoms in the formula.

What is the difference between an empirical formula and a molecular formula ?

An empirical formula gives the simplest whole-number ratio of atoms in a compound. A molecular formula gives the actual number of atoms of each element in one molecule of the compound.

Define relative isotopic mass

The relative isotopic mass is the mass of one atom of a specific isotope relative to 1/12 of the mass of a carbon-12 atom.

True or False? The relative atomic mass ( A r ) of an element is equal to the relative isotopic mass of its most abundant isotope.

False. The A r is the weighted mean of the relative isotopic masses of all isotopes of an element, taking into account their relative abundances.

Why is 1/12 of a carbon-12 atom used as the standard for relative atomic masses?

Carbon-12 provides a fixed, reproducible international standard. Its mass is defined as exactly 12 atomic mass units, giving a reference unit for comparing all other atomic masses.

Define relative atomic mass ( A r )

The relative atomic mass ( A r ) is the weighted mean mass of an atom of an element relative to 1/12 of the mass of a carbon-12 atom.

What distinguishes a compound from an element ?

A compound contains two or more different elements chemically joined together. An element consists of only one type of atom and cannot be broken down into simpler substances by chemical means.

Define relative formula mass ( M r )

The relative formula mass ( M r ) is the sum of the relative atomic masses of all atoms in one formula unit of a substance.

Define Avogadro constant ( N A )

The Avogadro constant ( N A ) is 6.02 × 10 23 mol -1 . It represents the number of particles (atoms, molecules, ions or electrons) in one mole of a substance.

The amount in moles ( n ) can be calculated using n = .......... where m is mass in grams and M is molar mass in g mol -1 .

The amount in moles ( n ) can be calculated using n = m / M where m is mass in grams and M is molar mass in g mol -1 .

A mole is the amount of substance containing the same number of fundamental units as there are atoms in exactly 12.00 g of carbon-12, equal to 6.02 × 10 23 particles.

True or False? One mole of any element has a mass in grams equal to its relative atomic mass .

True. One mole of any element equals its A r in grams. For example, one mole of carbon-12 has a mass of 12.00 g and one mole of water (H 2 O) has a mass of 18.0 g.

The molar mass ( M ) of a substance is the mass of one mole of that substance, measured in g mol -1 . It is numerically equal to the relative formula mass.

Why do chemists use moles rather than counting individual particles?

Atoms and molecules are too small to count individually. The mole links the measurable mass of a substance to the number of particles it contains via the Avogadro constant , enabling stoichiometric calculations.

True or False? The Avogadro constant applies only to atoms.

False. The Avogadro constant ( N A = 6.02 × 10 23 mol -1 ) applies to any type of particle, including atoms , molecules , ions and electrons .

How many moles are in 46.0 g of ethanol, C 2 H 5 OH? ( A r : C = 12.0, H = 1.0, O = 16.0)

Molar mass of C 2 H 5 OH = (2 × 12.0) + (6 × 1.0) + 16.0 = 46.0 g mol -1 Moles = mass ÷ molar mass = 46.0 ÷ 46.0 = 1.00 mol

What are spectator ions ?

Spectator ions are ions present in a reaction mixture that do not take part in the reaction. They appear unchanged on both sides of a full equation and are omitted from ionic equations.

In a balanced equation , the number of atoms of each element on the reactants side must equal the number on the .......... side.

In a balanced equation , the number of atoms of each element on the reactants side must equal the number on the products side.

True or False? Spectator ions are included in a net ionic equation.

False. Spectator ions do not participate in the reaction. They are cancelled from both sides, and only the reacting species appear in the net ionic equation .

How is a precipitation reaction identified from an equation?

A precipitation reaction involves two aqueous solutions reacting to form an insoluble solid product, identified by the state symbol (s) on a product in the equation.

What is an ionic equation ?

An ionic equation shows only the ions and other particles that take part in a reaction. Spectator ions are omitted.

Why are ionic equations more useful than full symbol equations?

Ionic equations show only the particles that actually react, removing spectator ions . This makes it easier to identify the essential chemistry and recognise the same reaction with different counter-ions.

True or False? Atoms are created and destroyed during a chemical reaction.

False. Atoms are neither created nor destroyed during a chemical reaction. This is why equations must be balanced : the total number of each type of atom is equal on both sides.

What are the typical products of a neutralisation reaction ?

A neutralisation reaction produces a salt and water . If a carbonate is the base, carbon dioxide is also produced. The ionic equation for a simple acid-base reaction is: H + (aq) + OH - (aq) → H 2 O (l).

To write the formula of an ionic compound, the charges of the cation and anion are balanced until the overall charge is .......... .

To write the formula of an ionic compound, the charges of the cation and anion are balanced until the overall charge is zero .

What is the method for calculating an empirical formula from mass data?

Find moles of each element: n = mass ÷ A r Divide all values by the smallest to get a ratio Scale up to whole numbers if needed The ratio gives the empirical formula .

Define water of crystallisation

Water of crystallisation is water chemically incorporated into the crystal structure of a compound. A compound containing it is called hydrated and is written with a dot, e.g. CuSO 4 •5H 2 O.

The ideal gas equation is PV = .......... where R = 8.31 J mol -1 K -1 .

The ideal gas equation is PV = nRT where R = 8.31 J mol -1 K -1 .

True or False? The percentage by mass of an element in a compound depends only on the A r of that element.

False. The percentage by mass also depends on the number of atoms of that element and the M r of the compound: % mass = ( A r × number of atoms) ÷ M r × 100.

What is the equation for percentage by mass of an element in a compound?

% mass of element = ( A r × number of atoms of element) ÷ M r of compound × 100 This gives the percentage contribution of that element to the total mass.

To find the molecular formula , divide the M r of the compound by the M r of the .......... formula, then multiply each atom count by this ratio.

To find the molecular formula , divide the M r of the compound by the M r of the empirical formula, then multiply each atom count by this ratio.

True or False? Heating a hydrated salt to constant mass converts it to an anhydrous compound.

True. Heating drives off the water of crystallisation , leaving behind the anhydrous salt. The mass lost can be used to calculate the number of moles of water per formula unit.

Why must units be converted before using PV = nRT ?

The equation requires SI units : pressure in Pa (not kPa), volume in m 3 (not cm 3 or dm 3 ) and temperature in Kelvin (not °C). Using wrong units gives an incorrect value for n or M r .

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Define empirical formula

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Define empirical formula
An empirical formula is the simplest whole-number ratio of atoms of each element in a compound or molecule.
The relative formula mass (M_r) is calculated by .......... the A_r values of all atoms in the formula.
The relative formula mass (M_r) is calculated by adding the A_r values of all atoms in the formula.
What is the difference between an empirical formula and a molecular formula?
An empirical formula gives the simplest whole-number ratio of atoms in a compound. A molecular formula gives the actual number of atoms of each element in one molecule of the compound.
Define relative isotopic mass
The relative isotopic mass is the mass of one atom of a specific isotope relative to 1/12 of the mass of a carbon-12 atom.
True or False?
The relative atomic mass (A_r) of an element is equal to the relative isotopic mass of its most abundant isotope.
False.
The A_r is the weighted mean of the relative isotopic masses of all isotopes of an element, taking into account their relative abundances.
Why is 1/12 of a carbon-12 atom used as the standard for relative atomic masses?
Carbon-12 provides a fixed, reproducible international standard. Its mass is defined as exactly 12 atomic mass units, giving a reference unit for comparing all other atomic masses.
Define relative atomic mass (A_r)
The relative atomic mass (A_r) is the weighted mean mass of an atom of an element relative to 1/12 of the mass of a carbon-12 atom.
What distinguishes a compound from an element?
A compound contains two or more different elements chemically joined together. An element consists of only one type of atom and cannot be broken down into simpler substances by chemical means.
Define relative formula mass (M_r)
The relative formula mass (M_r) is the sum of the relative atomic masses of all atoms in one formula unit of a substance.
Define Avogadro constant (N_A)
The Avogadro constant (N_A) is 6.02 × 10²³ mol^-1. It represents the number of particles (atoms, molecules, ions or electrons) in one mole of a substance.
The amount in moles (n) can be calculated using n = .......... where m is mass in grams and M is molar mass in g mol^-1.
The amount in moles (n) can be calculated using n = m/M where m is mass in grams and M is molar mass in g mol^-1.
What is a mole?
A mole is the amount of substance containing the same number of fundamental units as there are atoms in exactly 12.00 g of carbon-12, equal to 6.02 × 10²³ particles.
True or False?
One mole of any element has a mass in grams equal to its relative atomic mass.
True.
One mole of any element equals its A_r in grams. For example, one mole of carbon-12 has a mass of 12.00 g and one mole of water (H₂O) has a mass of 18.0 g.
Define molar mass
The molar mass (M) of a substance is the mass of one mole of that substance, measured in g mol^-1. It is numerically equal to the relative formula mass.
Why do chemists use moles rather than counting individual particles?
Atoms and molecules are too small to count individually. The mole links the measurable mass of a substance to the number of particles it contains via the Avogadro constant, enabling stoichiometric calculations.
True or False?
The Avogadro constant applies only to atoms.
False.
The Avogadro constant (N_A = 6.02 × 10²³ mol^-1) applies to any type of particle, including atoms, molecules, ions and electrons.
How many moles are in 46.0 g of ethanol, C₂H₅OH? (A_r: C = 12.0, H = 1.0, O = 16.0)
Molar mass of C₂H₅OH = (2 × 12.0) + (6 × 1.0) + 16.0 = 46.0 g mol^-1
Moles = mass ÷ molar mass = 46.0 ÷ 46.0 = 1.00 mol
What are spectator ions?
Spectator ions are ions present in a reaction mixture that do not take part in the reaction. They appear unchanged on both sides of a full equation and are omitted from ionic equations.
In a balanced equation, the number of atoms of each element on the reactants side must equal the number on the .......... side.
In a balanced equation, the number of atoms of each element on the reactants side must equal the number on the products side.
True or False?
Spectator ions are included in a net ionic equation.
False.
Spectator ions do not participate in the reaction. They are cancelled from both sides, and only the reacting species appear in the net ionic equation.
How is a precipitation reaction identified from an equation?
A precipitation reaction involves two aqueous solutions reacting to form an insoluble solid product, identified by the state symbol (s) on a product in the equation.
What is an ionic equation?
An ionic equation shows only the ions and other particles that take part in a reaction. Spectator ions are omitted.
Why are ionic equations more useful than full symbol equations?
Ionic equations show only the particles that actually react, removing spectator ions. This makes it easier to identify the essential chemistry and recognise the same reaction with different counter-ions.
True or False?
Atoms are created and destroyed during a chemical reaction.
False.
Atoms are neither created nor destroyed during a chemical reaction. This is why equations must be balanced: the total number of each type of atom is equal on both sides.
What are the typical products of a neutralisation reaction?
A neutralisation reaction produces a salt and water. If a carbonate is the base, carbon dioxide is also produced. The ionic equation for a simple acid-base reaction is: H⁺ (aq) + OH^- (aq) → H₂O (l).
To write the formula of an ionic compound, the charges of the cation and anion are balanced until the overall charge is ...........
To write the formula of an ionic compound, the charges of the cation and anion are balanced until the overall charge is zero.
What is the method for calculating an empirical formula from mass data?
1. Find moles of each element: n = mass ÷ A_r
2. Divide all values by the smallest to get a ratio
3. Scale up to whole numbers if needed
The ratio gives the empirical formula.
Define water of crystallisation
Water of crystallisation is water chemically incorporated into the crystal structure of a compound. A compound containing it is called hydrated and is written with a dot, e.g. CuSO₄•5H₂O.
The ideal gas equation is PV = .......... where R = 8.31 J mol^-1 K^-1.
The ideal gas equation is PV = nRT where R = 8.31 J mol^-1 K^-1.
True or False?
The percentage by mass of an element in a compound depends only on the A_r of that element.
False.
The percentage by mass also depends on the number of atoms of that element and the M_r of the compound: % mass = (A_r × number of atoms) ÷ M_r × 100.
What is the equation for percentage by mass of an element in a compound?
% mass of element = (A_r × number of atoms of element) ÷ M_r of compound × 100
This gives the percentage contribution of that element to the total mass.
To find the molecular formula, divide the M_r of the compound by the M_r of the .......... formula, then multiply each atom count by this ratio.
To find the molecular formula, divide the M_r of the compound by the M_r of the empirical formula, then multiply each atom count by this ratio.
True or False?
Heating a hydrated salt to constant mass converts it to an anhydrous compound.
True.
Heating drives off the water of crystallisation, leaving behind the anhydrous salt. The mass lost can be used to calculate the number of moles of water per formula unit.
Why must units be converted before using PV = nRT?
The equation requires SI units: pressure in Pa (not kPa), volume in m³ (not cm³ or dm³) and temperature in Kelvin (not °C). Using wrong units gives an incorrect value for n or M_r.

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