A student carries out an investigation to identify an unknown Group 2 metal, M\text{M}M.
The student analyses metal M\text{M}M using a back-titration technique:
Stage 1 The student adds 150.0 cm3 of 1.00 mol dm−3 HCl(aq)1.00\text{ mol dm}^{-3}\ \text{HCl(aq)}1.00 mol dm−3 HCl(aq) to a flask containing 1.00 g of metal M\text{M}M. An excess of acid is present to ensure all of the metal reacts completely. A redox reaction occurs, generating hydrogen gas and a solution containing metal M\text{M}M in the +2 oxidation state.
Stage 2 The resulting solution from Stage 1 is transferred and made up to 250.0 cm3 with distilled water in a volumetric flask.
Stage 3 A 25.00 cm3 sample of this diluted solution is titrated against 0.500 mol dm−3 NaOH(aq)0.500\text{ mol dm}^{-3}\ \text{NaOH(aq)}0.500 mol dm−3 NaOH(aq) to determine the amount of unreacted hydrochloric acid:
NaOH(aq)+HCl(aq)→NaCl(aq)+H2O(l) \text{NaOH(aq)} + \text{HCl(aq)} \rightarrow \text{NaCl(aq)} + \text{H}_2\text{O(l)} NaOH(aq)+HCl(aq)→NaCl(aq)+H2O(l)The titration is repeated to obtain concordant titres.
All burette readings are recorded to the nearest 0.05 cm3.
| 1 | 2 | 3 | |
|---|---|---|---|
| Final reading / cm3\text{cm}^3cm3 | 20.80 | 21.30 | 25.05 |
| Initial reading / cm3\text{cm}^3cm3 | 0.10 | 1.20 | 5.05 |
Analyse these results to determine the molar mass of metal M\text{M}M and identify the metal.
