Atoms, molecules and stoichiometry: Practice Questions

What is the correct definition of the unified atomic mass unit?

When \(6.90\) g of ethanol (\(\text{C}_2\text{H}_5\text{OH}\)) is oxidised, \(4.50\) g of ethanoic acid (\(\text{CH}_3\text{COOH}\)) is produced. What is the percentage yield of ethanoic acid?

Equation: \( \text{C}_2\text{H}_5\text{OH}(\text{l}) + 2[\text{O}] \rightarrow \text{CH}_3\text{COOH}(\text{l}) + \text{H}_2\text{O}(\text{l}) \)

The complete combustion of 0.200 g of an organic compound containing only carbon, hydrogen, and sulfur produced 0.293 g of \(\text{CO}_2\) and 0.120 g of \(\text{H}_2\text{O}\). What is the empirical formula of the compound?
(\(A_r\): C = 12.0, H = 1.0, S = 32.1, O = 16.0)

What is the mass of \(0.25\) moles of magnesium carbonate, \(\text{MgCO}_3\)?

(A_r: Mg = 24.3, C = 12.0, O = 16.0)

A compound contains \(40.0\%\) carbon, \(6.7\%\) hydrogen and \(53.3\%\) oxygen by mass. What is the empirical formula of the compound? (A_r: C = 12.0, H = 1.0, O = 16.0)

Calculate the relative formula mass, \(M_r\), of hydrated iron(II) sulfate, \(\text{FeSO}_4 \cdot 7\text{H}_2\text{O}\).
(\(A_r\): \(\text{Fe} = 55.8\), \(\text{S} = 32.1\), \(\text{O} = 16.0\), \(\text{H} = 1.0\))

Calculate the volume, in \(\text{dm}^3\), occupied by \(11.0 \text{ g}\) of carbon dioxide gas at room temperature and pressure (r.t.p.). (Assume \(1\) mole of gas occupies \(24.0 \text{ dm}^3\) at r.t.p.; \(A_r\): C = 12.0, O = 16.0)

A technician dilutes \(10.0 \text{ cm}^3\) of \(2.50 \text{ mol dm}^{-3}\) \(\text{HCl}\) to a final volume of \(250 \text{ cm}^3\). \(25.0 \text{ cm}^3\) of this diluted solution is then titrated against \(0.0500 \text{ mol dm}^{-3}\) \(\text{Na}_2\text{CO}_3\). Calculate the volume of \(\text{Na}_2\text{CO}_3\) required for complete neutralisation.

A \( 2.50 \) g sample of impure sodium carbonate (\(\text{Na}_2\text{CO}_3\)) was dissolved in distilled water to make exactly \( 250 \) cm\(^3\) of solution. A \( 25.0 \) cm\(^3\) portion of this solution required \( 22.40 \) cm\(^3\) of \( 0.150 \) mol dm\(^{-3}\) hydrochloric acid for complete neutralisation.

Equation: \( \text{Na}_2\text{CO}_3(\text{aq}) + 2\text{HCl}(\text{aq}) \rightarrow 2\text{NaCl}(\text{aq}) + \text{CO}_2(\text{g}) + \text{H}_2\text{O}(\text{l}) \)

(a) Calculate the number of moles of HCl used in the titration.
(b) Determine the mass of \( \text{Na}_2\text{CO}_3 \) present in the original \( 2.50 \) g sample.
(c) Calculate the percentage purity of the sodium carbonate sample.

A gaseous hydrocarbon, \( \text{C}_x\text{H}_y \), was burnt completely in excess oxygen. When \( 10 \) cm\(^3\) of the hydrocarbon was reacted, it required \( 50 \) cm\(^3\) of oxygen for complete combustion and produced \( 30 \) cm\(^3\) of carbon dioxide gas. All volumes were measured at the same temperature and pressure.

(a) Use the volume ratios to determine the values of \( x \) and \( y \).
(b) Write the molecular formula of the hydrocarbon.
(c) Construct the balanced chemical equation for this combustion reaction.