Cambridge IGCSE · Chemistry (0620)

The mole and the Avogadro constant: Practice Questions

5 multiple-choice questions marked as you go, and 5 written questions with worked solutions. All on The mole and the Avogadro constant.

10 questions25 marksFree, no account

Question 1

1 mark

Which value represents the number of particles in exactly one mole of a substance?
(This value is known as the Avogadro constant)

Question 2

1 mark

What is the volume occupied by \(0.25 \text{ mol}\) of oxygen gas, \(O_2\), at room temperature and pressure (r.t.p.)?
(The molar gas volume at r.t.p. is \(24 \text{ dm}^3/\text{mol}\))

Question 3

1 mark

Magnesium reacts with hydrochloric acid according to the following equation:
\(Mg(s) + 2HCl(aq) \to MgCl_2(aq) + H_2(g)\)
A student adds \(1.2 \text{ g}\) of magnesium ribbon to \(25 \text{ cm}^3\) of \(2.0 \text{ mol/dm}^3\) hydrochloric acid.
Which statement about this reaction is correct?
(Relative atomic mass, \(A_r\): \(Mg = 24\))

Question 4

1 mark

Ammonium nitrate, \(NH_4NO_3\), is a common nitrogenous fertiliser. What is the percentage by mass of nitrogen in this compound?
(Relative atomic masses, \(A_r\): \(H=1, N=14, O=16\))

Question 5

1 mark

A compound contains $$40.0\%$$ carbon, $$6.7\%$$ hydrogen, and $$53.3\%$$ oxygen by mass. If its relative molecular mass is $$60$$, what is its molecular formula?

(Relative atomic masses: C = 12, H = 1, O = 16)

Question 6

3 marks

A solution of sodium hydroxide, \(NaOH\), has a concentration of \(0.5\text{ mol/dm}^3\). Calculate its concentration in \(\text{g/dm}^3\).
(Relative atomic masses: \(Na = 23, O = 16, H = 1\))

Write your answer out first, then check it against the worked solution.

Question 7

4 marks

Calculate the percentage by mass of nitrogen in ammonium nitrate, \(NH_4NO_3\).
(Relative atomic masses: \(N = 14, H = 1, O = 16\))

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Question 8

5 marks

In the reaction \(Mg + 2HCl \to MgCl_2 + H_2\), \(6\text{ g}\) of magnesium is added to \(0.4\text{ moles}\) of hydrochloric acid. Identify the limiting reactant.
(Relative atomic mass: \(Mg = 24\))

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Question 9

4 marks

Sulfur trioxide, \(SO_3\), is an oxide formed during the Contact process.
(a) Calculate the relative formula mass, \(M_r\), of sulfur trioxide.
(Relative atomic masses: \(S = 32, O = 16\))

(b) Calculate the amount of substance, in moles, present in \(20\text{ g}\) of sulfur trioxide.

(c) State the value of the Avogadro constant and use it to calculate the number of molecules present in this \(20\text{ g}\) sample of sulfur trioxide.

Write your answer out first, then check it against the worked solution.

Question 10

4 marks

A sample of methane, \(CH_4\), contains \(4.515 \times 10^{22}\) molecules.
(a) Calculate the number of moles of methane present in this sample.
(b) Calculate the mass of this sample in grams.
(Relative atomic masses: C = 12.0, H = 1.0; Avogadro constant \(L = 6.02 \times 10^{23}\text{ mol}^{-1}\))

Write your answer out first, then check it against the worked solution.

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