Quantitative Chemistry
Quantitative chemistry is the arithmetic of chemical reactions: working out how much product a given mass of reactant can make, based on conservation of mass, the principle that no atoms are lost or made in a reaction. Relative formula mass (Mr), the sum of the relative atomic masses in a formula, bridges a balanced equation and the masses of reactants and products measured in grams.
Before you start
Make sure you're comfortable with these topics first:
Method
- Balance the equation before any calculation. An unbalanced equation gives the wrong ratio and every subsequent mark is lost.
- Calculate Mr by adding the relative atomic masses of every atom in the formula, multiplying out the brackets and the subscripts first, for example Ca(OH)2 = 40 + 2 x (16 + 1) = 74.
- Use conservation of mass to fill in a missing mass: total mass of reactants = total mass of products, so a missing reactant or product mass is found by subtraction.
- Explain an apparent change in mass by naming the gas: a reaction in an open flask that appears to lose mass has released a gas such as carbon dioxide; one that appears to gain mass has taken in a gas, usually oxygen from the air, which is why a metal gains mass when it burns.
- For mass calculations from an equation, work in three steps: convert the known mass to moles using moles = mass / Mr, use the balancing numbers to find the moles of the substance you want, then convert back using mass = moles x Mr.
- Round only at the end, and give the answer to the same number of significant figures as the least precise value in the question, with the correct unit.
Worked example
A student heats 4.8 g of magnesium in air until it fully reacts, forming magnesium oxide. Calculate the mass of magnesium oxide formed. Relative atomic masses: Mg = 24, O = 16.
- Write and balance the equation: 2Mg + O2 gives 2MgO.
- Calculate the moles of magnesium: moles = mass / Ar = 4.8 / 24 = 0.20 mol.
- Use the ratio from the equation: 2 mol of Mg gives 2 mol of MgO, a 1 to 1 ratio, so 0.20 mol of MgO is formed.
- Calculate the Mr of magnesium oxide: 24 + 16 = 40.
- Convert moles to mass: mass = moles x Mr = 0.20 x 40 = 8.0 g.
- Final answer: 8.0 g of magnesium oxide. The mass has increased because oxygen from the air has combined with the magnesium, which is consistent with conservation of mass in a closed system.
Practice questions
Try each question, then tap to reveal the answer.
Q1State the law of conservation of mass.Show answer
Answer: No atoms are lost or made during a chemical reaction, so the total mass of the products equals the total mass of the reactants.
Q2Calculate the Mr of calcium carbonate, CaCO3. (Ca = 40, C = 12, O = 16)Show answer
Answer: 40 + 12 + (3 x 16) = 100.
Q3Calculate the Mr of Ca(OH)2. (Ca = 40, O = 16, H = 1)Show answer
Answer: 40 + 2 x (16 + 1) = 40 + 34 = 74.
Q4A sealed flask containing 12.0 g of reactants is shaken and a reaction occurs. What is the total mass afterwards?Show answer
Answer: 12.0 g, because no matter can enter or leave a sealed flask.
Q5Magnesium is burned in an open crucible and the mass increases. Explain why.Show answer
Answer: Oxygen from the air combines with the magnesium to form magnesium oxide, so the mass of the solid increases by the mass of oxygen added.
Q6Calculate the number of moles in 20 g of calcium carbonate. (Mr = 100)Show answer
Answer: 20 / 100 = 0.2 mol.
Q7A student measures the mass of a beaker of hydrochloric acid and marble chips as they react and finds it falls. Explain why.Show answer
Answer: Carbon dioxide gas is produced and escapes from the open beaker, so the measured mass falls even though no mass is destroyed.
Exam-style questions
Written in the style of a GCSE Science exam paper, with a full mark scheme.
In a reaction, 8.0 g of methane reacts completely with 32.0 g of oxygen to produce 22.0 g of carbon dioxide and water. Calculate the mass of water produced, and state the principle you used.
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Calculate the mass of calcium oxide produced when 25 g of calcium carbonate is completely decomposed. CaCO3 gives CaO + CO2. Relative formula masses: CaCO3 = 100, CaO = 56.
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Free printable worksheet
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This topic is chapter 6 of GCSE Chemistry Workbook, the whole course as one free printable PDF.
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