Atoms, molecules and stoichiometry: Practice Questions

What is the oxidation number of sulfur in the \( \text{S}_2\text{O}_3^{2-} \) ion?

What is the mass of magnesium chloride, \( MgCl_2 \), formed when \( 0.050 \text{ mol} \) of magnesium carbonate reacts with excess hydrochloric acid?
\( MgCO_3 + 2HCl \rightarrow MgCl_2 + H_2O + CO_2 \)
(\( A_r: Mg = 24.3, Cl = 35.5 \))

A \( 5.00 \, \text{g} \) sample of hydrated copper(II) sulfate, \( \text{CuSO}_4 \cdot x\text{H}_2\text{O} \), is heated until all water of crystallisation is removed. The mass of the anhydrous salt remaining is \( 3.20 \, \text{g} \).
What is the value of \( x \)?
(\( M_r: \text{CuSO}_4 = 159.6, \text{H}_2\text{O} = 18.0 \))

What is the correct formula for iron(III) sulfate?

Excess silver nitrate is added to \( 25.0 \, \text{cm}^3 \) of a \( 0.100 \, \text{mol dm}^{-3} \) solution of magnesium chloride.
\( \text{MgCl}_2(aq) + 2\text{AgNO}_3(aq) \rightarrow \text{Mg}(\text{NO}_3)_2(aq) + 2\text{AgCl}(s) \)
What is the mass of silver chloride precipitated?
(\( M_r: \text{AgCl} = 143.4 \))

Define the term relative atomic mass, \( A_r \), with reference to the unified atomic mass unit.

Calculate the volume, in \(\text{dm}^3\), occupied by \(0.25\text{ moles}\) of oxygen gas at room temperature and pressure (r.t.p.), assuming the molar volume of a gas is \(24.0\text{ dm}^3\text{ mol}^{-1}\).

A \(5.00\text{ g}\) sample of hydrated barium chloride, \(BaCl_2 \cdot xH_2O\), was heated until all water was lost. The mass of the anhydrous salt remaining was \(4.26\text{ g}\). Determine the value of \(x\). [\(M_r\): \(BaCl_2 = 208.3\); \(H_2O = 18.0\)]

(a) Define the term mole with reference to the Avogadro constant. (1)

(b) An organic compound is found to contain \( 40.0\% \) carbon, \( 6.7\% \) hydrogen, and \( 53.3\% \) oxygen by mass. Calculate the empirical formula of this compound. [Relative atomic masses, \( A_r \): C, 12.0; H, 1.0; O, 16.0] (3)

A \( 10 \text{ cm}^3 \) sample of a gaseous hydrocarbon, \( C_xH_y \), is reacted with \( 70 \text{ cm}^3 \) of oxygen, which is an excess. After combustion and cooling to room temperature, the total volume of gas remaining is \( 50 \text{ cm}^3 \). When this gas mixture is passed through aqueous sodium hydroxide, the volume decreases to \( 20 \text{ cm}^3 \). All gas volumes are measured at the same temperature and pressure.
(a) Deduce the volume of \( CO_2 \) produced in the reaction.
(b) Calculate the volume of oxygen that reacted with the hydrocarbon.
(c) Use your answers to deduce the values of \( x \) and \( y \) and state the molecular formula of the hydrocarbon.