Fundamentals of chemistry: practice with explanations

Web companion updated November 2, 2026

Try these practice questions before opening the document below. Write a reason for each answer, then compare your reasoning with the explanation. These are additional revision exercises, rather than a transcription of the textbook review.

1. Is oxygen gas a compound?

Answer: no. O₂ contains two atoms in each molecule, but both are oxygen. A compound must contain different elements. Count the types of element, rather than the number of atoms, when deciding between an element and a compound.

2. Why can salt water have different compositions?

Answer: it is a mixture. You can dissolve different quantities of salt in a given quantity of water, within its solubility limit. A pure compound has a definite chemical composition. A clear, uniform sample can still be a mixture, so appearance is insufficient evidence of purity.

3. Find the particles in a magnesium-24 ion, Mg²⁺, with Z = 12

Working: protons = 12; neutrons = 24 − 12 = 12; electrons = 12 − 2 = 10. The 2+ charge means that two electrons were lost. As a check, 12 positive charges and 10 negative charges leave a net charge of +2. The nucleus retains its identity as magnesium.

4. How do carbon-12 and carbon-14 differ?

Answer: both have six protons, but carbon-12 has six neutrons while carbon-14 has eight. They are isotopes of the same element. Changing neutron number changes the mass number; changing proton number would identify a different element. Their neutral atoms both have six electrons.

5. Count the atoms represented by 2Al₂(SO₄)₃

Working: first expand one formula unit: Al = 2, S = 3 and O = 4 × 3 = 12. Then multiply every count by the coefficient 2: Al = 4, S = 6 and O = 24. The expression represents 34 atoms in total. A frequent mistake is applying the coefficient only to aluminium.

6. Calculate the amount in 8.8 g of carbon dioxide

Working: using C = 12 and O = 16, the molar mass of CO₂ is 12 + 2 × 16 = 44 g/mol. Therefore n = 8.8 g ÷ 44 g/mol = 0.20 mol of CO₂ molecules. The gram units cancel, leaving moles. This amount contains 0.20 mol of carbon atoms and 0.40 mol of oxygen atoms.

7. How many molecules are in 0.050 mol of CO₂?

Working: multiply the amount by the Avogadro constant: 0.050 × 6.02214076 × 10²³ = approximately 3.0 × 10²² molecules, to two significant figures. Each molecule has three atoms, so the total atom count would be three times as large. Label the answer to make that distinction explicit.

Use your mistakes to choose what to revise

If questions 1–2 were difficult, return to classification of matter. For questions 3–4, practise separating proton number, neutron number and electron count. For questions 5–7, write the formula counts and units on separate lines before calculating. Repeat the questions without looking at the working, then use the original review document for additional course-specific practice.

Original review questions document

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