The Mole Concept and Stoichiometry: Practice Questions

What is the total number of atoms present in one mole of water molecules, \(\text{H}_2\text{O}\)?
(Avogadro constant, \(L = 6.02 \times 10^{23}\text{ mol}^{-1}\))

What volume of dilute hydrochloric acid of concentration \(0.200\text{ mol/dm}^3\) is completely neutralised by \(30.0\text{ cm}^3\) of aqueous sodium hydroxide of concentration \(0.100\text{ mol/dm}^3\)?

\(\text{NaOH(aq)} + \text{HCl(aq)} \rightarrow \text{NaCl(aq)} + \text{H}_2\text{O(l)}\)

When \(4.99\text{ g}\) of hydrated copper(II) sulfate crystals, \(\text{CuSO}_4 \cdot x\text{H}_2\text{O}\), is heated strongly to constant mass, \(3.19\text{ g}\) of anhydrous copper(II) sulfate, \(\text{CuSO}_4\), remains.

What is the value of the integer \(x\)?
(Relative atomic masses: \(\text{H} = 1.0\), \(\text{O} = 16.0\), \(\text{S} = 32.1\), \(\text{Cu} = 63.5\))

What mass of sodium hydroxide, \(\text{NaOH}\), is required to prepare \(500\text{ cm}^3\) of a \(0.200\text{ mol/dm}^3\) aqueous solution?
(Relative atomic masses: \(\text{H} = 1.0\), \(\text{O} = 16.0\), \(\text{Na} = 23.0\))

What volume of dilute nitric acid of concentration \(0.250\text{ mol/dm}^3\) is completely neutralised by \(30.0\text{ cm}^3\) of aqueous sodium hydroxide of concentration \(0.500\text{ mol/dm}^3\)?

\(\text{NaOH(aq)} + \text{HNO}_3\text{(aq)} \rightarrow \text{NaNO}_3\text{(aq)} + \text{H}_2\text{O(l)}\)

Calculate the relative molecular mass (\( M_r \)) of ethanol, \( \text{CH}_3\text{CH}_2\text{OH} \).
(Relative atomic masses: \( \text{H} = 1 \), \( \text{C} = 12 \), \( \text{O} = 16 \))

Determine the empirical formula of a compound that contains 80% carbon and 20% hydrogen by mass.
(Relative atomic masses: \( \text{C} = 12 \), \( \text{H} = 1 \))

A sample of \( 4.8 \text{ g} \) of magnesium ribbon is reacted with excess dilute sulfuric acid. Calculate the volume of hydrogen gas produced at room temperature and pressure.
(Relative atomic mass: \( \text{Mg} = 24 \); Molar volume of gas = \( 24 \text{ dm}^3 \))

A sample containing \(5.30\text{ g}\) of pure sodium carbonate, \(\text{Na}_2\text{CO}_3\), is completely dissolved in distilled water to make \(250\text{ cm}^3\) of standard solution.

[Relative atomic masses: \(A_r(\text{C}) = 12.0\), \(A_r(\text{O}) = 16.0\), \(A_r(\text{Na}) = 23.0\); Molar volume of gas at r.t.p. \(= 24.0\text{ dm}^3\text{ mol}^{-1}\)]

(a) Calculate the concentration of the sodium carbonate solution in \(\text{mol dm}^{-3}\).
(b) A \(25.0\text{ cm}^3\) portion of this sodium carbonate solution is reacted with excess dilute sulfuric acid according to the equation:
\(\text{Na}_2\text{CO}_3\text{(aq)} + \text{H}_2\text{SO}_4\text{(aq)} \rightarrow \text{Na}_2\text{SO}_4\text{(aq)} + \text{H}_2\text{O(l)} + \text{CO}_2\text{(g)}\)
Calculate the volume of carbon dioxide gas produced, in \(\text{cm}^3\), measured at room temperature and pressure (r.t.p.).

A student investigates the purity of a sample of hydrated sodium carbonate, \(Na_{2}CO_{3} \cdot xH_{2}O\).
(a) A 7.15 g sample of the hydrated salt is dissolved in distilled water to make 250 cm\(^{3}\) of solution. 25.0 cm\(^{3}\) of this solution required 20.0 cm\(^{3}\) of 0.125 mol/dm\(^{3}\) hydrochloric acid for complete neutralisation. Calculate the number of moles of sodium carbonate in 250 cm\(^{3}\) of the solution.
The equation for the reaction is:
\(Na_{2}CO_{3}(aq) + 2HCl(aq) \rightarrow 2NaCl(aq) + H_{2}O(l) + CO_{2}(g)\)
(b) Using your answer from (a), calculate the relative formula mass (\(M_{r}\)) of the hydrated salt and determine the integer value of \(x\).
(c) In a separate experiment, a 5.00 g sample of impure anhydrous sodium carbonate was reacted with excess sulfuric acid. If 1.08 dm\(^{3}\) of carbon dioxide gas was collected at room temperature and pressure, calculate the percentage purity of the anhydrous sodium carbonate. [Relative atomic masses: \(H=1, C=12, O=16, Na=23\); Molar volume of gas at r.t.p. = 24 dm\(^{3}\)/mol]