OxfordAQA International GCSE Chemistry 3.6 Quantitative Chemistry Exam Questions

About OxfordAQA International GCSE Chemistry 3.6 Quantitative Chemistry

Quantitative Chemistry is the calculation core of OxfordAQA International GCSE Chemistry, and its skills appear in questions across every other topic. You need to balance equations with state symbols, use conservation of mass, calculate relative formula mass and percentage by mass, and find empirical and molecular formulae. The mole, Avogadro's constant (6.02 x 10^23), reacting masses, concentration in mol/dm3 and g/dm3, and titrations with strong acids and alkalis follow, along with the molar gas volume of 24 dm3 at room temperature and pressure. Examiners award method marks, so always show working, and they expect answers to sensible significant figures with units.

What you need to know

• Equations and conservation of mass Write balanced symbol equations with state symbols (s), (l), (g) and (aq). Mass is conserved because no atoms are created or destroyed; apparent changes happen when a gas escapes or is gained from the air.
• Relative formula mass Mr is the sum of the relative atomic masses of all atoms in the formula; Mr of CaCO3 = 40 + 12 + (3 x 16) = 100. Percentage by mass of an element = (Ar x number of atoms) / Mr x 100.
• Empirical formula Divide the mass or percentage of each element by its Ar, then divide by the smallest answer to get the simplest whole number ratio. The molecular formula is a multiple of the empirical formula found using Mr.
• The mole One mole of a substance is its Mr in grams and contains 6.02 x 10^23 particles. Moles = mass / Mr. Use mole ratios from the balanced equation to find reacting masses.
• Yield The actual yield is often less than the calculated amount because the reaction may be reversible, product is lost when separated, or side reactions occur.
• Concentration Concentration (mol/dm3) = moles / volume (dm3). Divide cm3 by 1000 to get dm3. Multiply mol/dm3 by Mr to get g/dm3.
• Titration Required practical: find the concentration of an unknown strong acid (hydrochloric, nitric or sulfuric) by titration with a strong base. Use a pipette for the fixed volume, a burette and indicator, and repeat for concordant results.
• Gas volumes One mole of any gas occupies 24 dm3 at room temperature and pressure, so volume = moles x 24.

Common mistakes in 3.6 Quantitative Chemistry

• Forgetting to convert cm3 to dm3 in concentration calculations. Divide by 1000 first.
• Using the mass of the whole equation instead of mole ratios. Convert mass to moles, use the ratio from the balanced equation, then convert back.
• Rounding ratios such as 1.5 to 2 in empirical formula questions. Multiply all values by 2 to get whole numbers.
• Changing small numbers inside formulae to balance equations. Only add big numbers in front of formulae.
• Ignoring the mole ratio in titrations with sulfuric acid. H2SO4 reacts with 2NaOH, so the ratio is 1:2.

Try three 3.6 Quantitative Chemistry exam-style questions