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Practice questions with answers

These 24 questions go from finding a molar mass to using it in equations. Try each one on paper first, then open the answer. All answers use the IUPAC abridged atomic weights; with whole-number school values your figure may differ in the last digit, which is fine.

How to use this page

Work in order. Questions 1 to 4 are about counting atoms in a formula. Questions 5 to 11 change between grams, moles and particles. Questions 12 to 17 are on percentage composition and formulas. The rest use molar mass in solutions, gases and equations. If a group goes wrong, the linked guide explains the method.

  1. Find the molar mass of calcium nitrate, Ca(NO3)2.

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    1 Ca, 2 N, 6 O: 40.078 + 2 × 14.007 + 6 × 15.999 = 164.09 g/mol.

  2. Find the molar mass of ammonium phosphate, (NH4)3PO4.

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    3 N, 12 H, 1 P, 4 O: 149.09 g/mol.

  3. Find the molar mass of ethanoic acid, CH3COOH.

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    2 C, 4 H, 2 O: 60.05 g/mol.

  4. Find the molar mass of Mohr's salt, FeSO4·(NH4)2SO4·6H2O.

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    Add the three parts: 151.90 + 132.13 + 6 × 18.015 = 392.12 g/mol.

  5. How many moles are in 36 g of water?

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    36 ÷ 18.02 = 1.998 mol.

  6. How many moles are in 10.6 g of anhydrous sodium carbonate?

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    10.6 ÷ 105.99 = 0.1 mol.

  7. What is the mass of 0.25 mol of glucose?

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    0.25 × 180.16 = 45.04 g.

  8. What is the mass of 3 mol of carbon dioxide?

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    3 × 44.01 = 132 g.

  9. How many molecules are in 8.8 g of carbon dioxide?

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    8.8 ÷ 44.01 = 0.2 mol; × 6.022 × 1023 = 1.204 × 1023 molecules.

  10. How many atoms are in 4 g of helium?

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    4 ÷ 4.003 = 0.9994 mol of atoms; that is 6.018 × 1023 atoms.

  11. Which has more molecules: 10 g of hydrogen or 100 g of oxygen?

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    Hydrogen: 10 ÷ 2.016 = 4.96 mol. Oxygen: 100 ÷ 32.00 = 3.125 mol. The hydrogen has more.

  12. Find the percentage of nitrogen in ammonium nitrate, NH4NO3.

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    2 × 14.007 ÷ 80.04 × 100 = 35.0%.

  13. Find the percentage of oxygen in calcium carbonate.

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    3 × 15.999 ÷ 100.09 × 100 = 48.0%.

  14. Find the percentage of water in Epsom salt, MgSO4·7H2O.

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    7 × 18.015 ÷ 246.47 × 100 = 51.2%.

  15. How much iron is in 1 kg of iron(III) oxide?

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    Iron is 69.94% of Fe2O3, so 1000 g holds 699 g of iron.

  16. A compound has 27.3% carbon and 72.7% oxygen. Find its empirical formula.

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    C: 27.3 ÷ 12.011 = 2.27. O: 72.7 ÷ 15.999 = 4.54. Ratio 1 : 2, so CO2.

  17. A hydrocarbon has the empirical formula CH and a molar mass of about 78 g/mol. Find its molecular formula.

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    CH has a mass of 13.02. 78 ÷ 13.02 = 6, so the formula is C6H6, benzene.

  18. What mass of sodium hydroxide is needed for 500 mL of 0.2 M solution?

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    0.2 × 0.5 × 40.00 = 4 g.

  19. 4.9 g of sulphuric acid is dissolved and made up to 250 mL. Find the molarity.

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    Moles: 4.9 ÷ 98.07 = 0.04996. Molarity: ÷ 0.250 = 0.2 M.

  20. What volume does 8 g of oxygen occupy at 0 °C and 1 atm?

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    8 ÷ 32.00 = 0.25 mol; × 22.4 = 5.6 litres.

  21. A gas has a vapour density of 32. What is its molar mass, and which common gas could it be?

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    Molar mass = 2 × 32 = 64 g/mol. Sulphur dioxide has a molar mass of 64.06.

  22. What mass of carbon dioxide forms when 25 g of calcium carbonate is heated strongly?

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    Moles of CaCO3: 25 ÷ 100.09 = 0.2498. One mole gives one mole of CO2: 0.2498 × 44.01 = 11.0 g.

  23. What mass of water forms when 8 g of methane burns completely? (CH4 + 2O2 → CO2 + 2H2O)

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    Moles of methane: 8 ÷ 16.04 = 0.4987. Water: twice that, 0.9973 mol; × 18.02 = 18.0 g.

  24. 2.86 g of washing soda crystals, Na2CO3·10H2O, are heated until all the water is lost. What mass remains?

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    Moles: 2.86 ÷ 286.14 = 0.009995. Mass of anhydrous salt: × 105.99 = 1.06 g.

Where to read the method

These questions are our own. They are not taken from any textbook or examination paper.

Which values? This site uses the abridged standard atomic weights published by IUPAC (CIAAW), read on 12 November 2026. Your textbook or question paper may round them differently, for example H = 1 and O = 16, so its answer can differ slightly. In an exam, use the values the paper gives.

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What is molar mass? All guides