AQA GCSE C3 Quantitative Chemistry

Law of conservation of mass

The law of conservation of mass states that no atoms are lost or made during a chemical reaction so the mass of the products equals the mass of the reactants.

Balanced equation of magnesium reacting with hydrochloric acid

Mg(s) + 2 HCl(aq) → MgCl2(aq) + H2(g)

Relative atomic mass (RAM)

Average mass of atoms in an element taking into account masses and abundance of its isotopes, relative to 12C.

Relative formula mass (RFM)

Sum of RAM's of all atoms in the formula.

Relative formula mass of CaF2

CaF2 - (Ar values: Ca = 40, F = 19) 40 + 19 + 19 = 78

Relative formula mass of C6H12O6

C6H12O6 - (Ar values: C = 12, H = 1, O = 16) (12 x 6) + (1 x 12) + (16 x 6) = 180

Escape of gases in a reaction

They are both gases.

Mean mass of magnesium produced

(3.3 + 3.5 + 3.2) / 3 = 3.3

Increasing precision of results

Measure to more decimal places or use a more sensitive balance/apparatus.

Avogadro's constant

The number of atoms, molecules or ions in a mole of a given substance. The value of the constant is 6.02 x 10^23.

Formula linking mass, molecular mass and moles

Mass = Mr x Moles.

Mass of 20 moles of calcium carbonate (CaCO3)

Mr = 100; 100 x 20 = 2000 g.

Amount of carbon dioxide in moles from 0.32 g

Relative atomic masses (Ar): carbon = 12, oxygen = 16.

Relative atomic mass (Ar)

A measure of the mass of an atom relative to the mass of carbon-12, e.g., carbon = 12, oxygen = 16.

Moles

A quantity of substance that contains as many entities as there are atoms in 12 grams of carbon-12, calculated as Mass / Mr.

Balanced equation for ammonia formation

N2(g) + 3 H2(g) ⇌ 2 NH3(g)

Mass of nitrogen needed for ammonia

To form 6.8 tonnes of ammonia, 5.6 tonnes of nitrogen is required.

Molar mass of ammonia (Mr)

The molar mass of ammonia is 17 g/mol.

Moles of ammonia from mass

6800000 g of ammonia corresponds to 400000 moles.

Ratio of nitrogen to ammonia

The ratio of nitrogen to ammonia in the reaction is 1:2.

Molar mass of nitrogen (N2)

The molar mass of nitrogen gas (N2) is 28 g/mol.

Limiting reactant

The reactant that is completely used up in a chemical reaction, limiting the amount of products formed.

Decomposition of hydrogen peroxide

Hydrogen peroxide decomposes to form water and oxygen.

Balanced equation for hydrogen peroxide decomposition

2 H2O2(l) → 2 H2O + O2(g)

Molar mass of hydrogen peroxide (H2O2)

The molar mass of hydrogen peroxide is 34 g/mol.

Moles of oxygen from hydrogen peroxide

From 40.8 g of hydrogen peroxide, 0.6 moles of oxygen gas are produced.

Mass of oxygen from moles

The mass of oxygen produced from 0.6 moles is 19.2 g.

Concentration formula (mass)

Concentration (g per dm3) = Mass (g) / Volume (dm3)

Concentration formula (moles)

Concentration (mol per dm3) = Number of moles / Volume (dm3)

Volume of potassium hydroxide solution

31.0 cm3 of potassium hydroxide solution neutralised 25.0 cm3 of 2.0 moldm−3 nitric acid.

HNO3 + KOH → KNO3 + H2O

The balanced chemical equation representing the reaction between nitric acid and potassium hydroxide.

Concentration of potassium hydroxide solution

Calculated in moldm−3.

Moles of HNO3 used

Calculated as Concentration x volume; 2 x 0.025 dm3 = 0.05 moles.

Moles of KOH

Calculated based on the 1:1 ratio with HNO3, resulting in 0.05 moles.

Concentration of KOH

Calculated using the formula Volume = Moles/concentration; 0.05 / 0.031 = 1.61.

Molar volume of a gas

1 mole of a gas at room temperature and pressure occupies 24 dm3.

Titration

A technique for finding the concentration of a solution by reacting a known volume of this solution with a solution of known concentration.

Conducting a titration

Steps include rinsing the pipette, adding an indicator, rinsing the burette, adding the solution gradually, recording the end point, and performing calculations.

Concordant volume results

Results that lie close to each other in titration measurements.

Theoretical amount of product

The maximum expected yield of product based on stoichiometric calculations.

Reasons for not obtaining theoretical yield

Reactions may not go to completion, products may be lost, or side reactions may occur.

Percentage yield

% Yield = (Actual mass of a product / Maximum theoretical mass of product) x 100%.

Calculating percentage yield of NH3

If 40.5 g NH3 is produced from 20.0 mol H2, the theoretical yield is calculated from the balanced equation N2 + 3 H2 → 2 NH3.

Theoretical amount of NH3 from H2

Calculated as 20/1.5 = 13.3 moles, leading to 13.3 x 17 (Mr of NH3) = 227 g.

Percentage yield of NH3 calculation

Calculated as 40.5/227 x 100 = 17.8%.

Atom economy

A measure of the amount of starting materials that end up as useful products, calculated as a ratio of the relative formula mass of desired product to the sum of relative formula masses of reactants.

Reaction I for CuCl2 production

CuCO3(s) + 2 HCl(aq) → CuCl2(aq) + H2O(l) + CO2(g).

Reaction II for CuCl2 production

CuO(s) + 2 HCl(aq) → CuCl2(aq) + H2O(l).

Better atom economy reaction

Reaction II has a better atom economy with a total formula mass of reactants = 152.5 and formula mass of CuCl2 = 134.5, leading to (134.5/152.5) x 100% = 88.2%.