Chemistry 9701 · AS & A Level · Analytical techniques
Analytical techniques — practice question
Washing soda is hydrated sodium carbonate, $\text{Na}_2\text{CO}_3 \cdot x\text{H}_2\text{O}$. A student set out to find the value of $x$ by titration, and obtained these results: $5.13\,\text{g}$ of washing soda crystals were dissolved in water and the solution was made up to $250\,\text{cm}^3$ in a standard volumetric flask. Then $25.0\,\text{cm}^3$ of this solution reacted exactly with $35.8\,\text{cm}^3$ of $0.100\,\text{mol dm}^{-3}$ hydrochloric acid, with carbon dioxide being produced.
(a(i))
Write a balanced equation for the reaction of $\text{Na}_2\text{CO}_3$ with HCl.
(a(ii))
Calculate the amount, in moles, of HCl in the $35.8\,\text{cm}^3$ of solution used in the titration.
(a(iii))
Use your answers to (i) and (ii) to work out the amount, in moles, of $\text{Na}_2\text{CO}_3$ in the $25.0\,\text{cm}^3$ of solution used in the titration.
(a(iv))
Use your answer to (iii) to calculate the mole amount of $\text{Na}_2\text{CO}_3$ in the $250\,\text{cm}^3$ of solution in the standard volumetric flask.
(a(v))
Hence calculate the mass of $\text{Na}_2\text{CO}_3$ contained in $5.13\text{ g}$ of washing soda crystals.
(b)[2]
Use your calculations in (a) to work out the value of $x$ in $\text{Na}_2\text{CO}_3 \cdot x\text{H}_2\text{O}$.
Worked solution & mark scheme
This 2-mark question has a full step-by-step worked solution and mark scheme. One marking point: “Therefore, $\mathrm{Na_2CO_3 + 2HCl \rightarrow 2NaCl + H_2O + CO_2}$” …