How many moles are in 21 g of a compound? What mass of product can 4 g of a reactant make? Moles questions look long, but every one of them uses the same few steps, and the steps are what this quiz practises.
The quiz begins with the skills every student needs, such as relative formula mass, percentage by mass and conservation of mass, then moves on to moles, reacting masses and limiting reactants. Most of the moles work is Higher tier, so those questions are flagged and Foundation students can leave them out. Every question that needs relative atomic masses prints the ones to use, and every answer comes with a short explanation.
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The balanced equation is CH4 + 2O2 → CO2 + 2H2O. What mass of H2O is made from 4 g of CH4, assuming the reaction goes to completion? Use these relative atomic masses (Ar): C = 12, H = 1, O = 16. Answers are given to 3 significant figures.
Answer: B, 9.00 g
Moles of CH4 = 4 ÷ 16 = 0.25 mol. The equation gives 1 : 2 for CH4 : H2O, so moles of H2O = 0.500 mol. Mass = moles × Mr = 0.500 × 18 = 9.00 g.
The balanced equation is 2Ca + O2 → 2CaO. A mixture contains 8.0 g of calcium and 1.6 g of oxygen. What is the maximum mass of calcium oxide that can form? Use these relative atomic masses (Ar): O = 16, Ca = 40. Answers are given to 3 significant figures.
Answer: C, 5.60 g
Moles of Ca = 8.0 ÷ 40 = 0.2 mol. Moles of O2 = 1.6 ÷ 32 = 0.05 mol. The equation needs 2 mol of Ca for each 1 mol of O2, so 0.05 mol of oxygen would need only 0.1 mol of calcium. Calcium is in excess; oxygen is limiting. Moles of CaO = 2 × 0.05 = 0.1 mol, so the mass is 0.1 × 56 = 5.60 g.
Almost every question follows the same order. Write down what you are given and what you are asked for, with units. Work out the relative formula mass (Mr) by adding the relative atomic masses of every atom in the formula. Turn the mass you know into moles with moles = mass ÷ Mr. If the question involves an equation, use the balancing numbers to move from the moles you have to the moles you want. Finally turn those moles into the quantity asked for: multiply by Mr for a mass, or by 24 dm³ for the volume of a gas when that value is given.
Writing the steps out in full feels slow, but it is the quickest way to find a slip. Each line is a small calculation, and a wrong answer is almost always one wrong line.
Moles are worked out from a mass in grams, so a mass in kilograms must be multiplied by 1000 first. Concentration questions use dm³, so a volume in cm³ is divided by 1000. Do these conversions at the start, before anything else.
Every calculation in this quiz that needs relative atomic masses states the ones to use, so use those values and not a different table. Printed tables are not identical: some give chlorine as 35.5 and others round other elements to one decimal place, which changes the last digits of an answer. Each question also says how its answers are rounded, usually to 3 significant figures. Keep every digit on the calculator until the last step and round once at the end.
When two reactants are mixed, work out the moles of each. Then use the equation to ask how much of one reactant the other would need. If you do not have enough of the first, it is the limiting reactant and it sets the amount of product. Use the limiting reactant, never the one in excess, for the rest of the calculation. Having equal amounts in moles does not mean both run out together: that is true only when the ratio in the equation is 1 : 1.
It is written to fit all three. Relative formula mass, moles, equation calculations and limiting reactants are in the GCSE Chemistry specifications of AQA, Pearson Edexcel and OCR. Percentage by mass is named by AQA and Edexcel, while OCR lists it only among its maths skills. Each calculation tells you which relative atomic masses to use, so it works whichever table you are given in the exam. Combined Science specifications were not checked for this quiz, so ask your teacher what your own course includes.
Most of them. Moles, the Avogadro constant and limiting reactants are Higher tier on all three boards. Using an equation to find masses is Higher tier on two of them and not marked Higher on the third. Relative formula mass, percentage by mass and conservation of mass are for everyone. Because it takes only one board to mark something Higher, the quiz flags a question as Higher whenever any of the three does. If you choose Foundation, you get the smaller set of questions that suit both tiers; if you choose Higher, you get everything.
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These practice questions were written by Cloudia Tutoring. They are not official AQA, Pearson Edexcel or OCR material, and Cloudia Tutoring is not affiliated with or endorsed by any exam board or the Department for Education. Always check your own exam board's specification.
Reviewed by Claudia Edwards on 20 October 2026 Last updated .
