Sodium sulfite is oxidised when it reacts with excess iodine. $$\[ \mathrm{Na}_{2} \mathrm{SO}_{3}(\mathrm{aq})+\mathrm{I}_{2}(\mathrm{aq})+\mathrm{H}_{2} \mathrm{O}(\mathrm{I}) \rightarrow \mathrm{Na}_{2} \mathrm{SO}_{4}(\mathrm{aq})+2 \mathrm{HI}(\mathrm{aq}) \]$$ The remaining iodine is then titrated using aqueous sodium thiosulfate. $$\[ \mathrm{I}_{2}(\mathrm{aq})+2 \mathrm{Na}_{2} \mathrm{~S}_{2} \mathrm{O}_{3}(\mathrm{aq}) \rightarrow \mathrm{Na}_{2} \mathrm{~S}_{4} \mathrm{O}_{6}(\mathrm{aq})+2 \mathrm{NaI}(\mathrm{aq}) \]$$ You will determine the integer value of $$\(x\)$$ in the formula of hydrated sodium sulfite, $$\(\mathrm{Na}_{2} \mathrm{SO}_{3} \cdot \mathrm{HH}_{2} \mathrm{O}\)$$, by titration. FA 4 is aqueous sodium thiosulfate containing $$\(14.24 \mathrm{~g}_{\text {of }} \mathrm{Na}_{2} \mathrm{~S}_{2} \mathrm{O}_{3}\)$$ in $$\(1.00 \mathrm{dm}^{3}\)$$. FA 5 is aqueous iodine, prepared as shown. - $$\(\quad 5.00 \mathrm{~g}\)$$ of hydrated sodium sulfite is added to $$\(600 \mathrm{~cm}^{3}\)$$ of $$\(0.100 \mathrm{~mol} \mathrm{dm}^{-3}\)$$ aqueous iodine. - The mixture is allowed to stand to ensure that all the sodium sulfite has been oxidised. - The mixture containing the remaining iodine is made up to $$\(1.00 \mathrm{dm}^{3}\)$$ with distilled water. FA 6 is starch indicator. Calculations (i) Give your answers to (c)(ii), (c)(iii), (c)(iv) and (c)(v) to an appropriate number of significant figures. (ii) Calculate the amount, in mol, of sodium thiosulfate present in the volume of FA 4 in (b). amount of $$\(\mathrm{Na}_{2} \mathrm{~S}_{2} \mathrm{O}_{3}=\)$$ .......................... $$\(\mathrm{mol}\)$$ (iii) Calculate the amount, in mol, of iodine in $$\(1.00 \mathrm{dm}^{3}\)$$ of FA 5 . $$\[ \text { amount of } \mathrm{I}_{2} \text { in } 1.00 \mathrm{dm}^{3}= \]$$ .......................... $$\(\mathrm{mol}\)$$ (iv) Use the information given and your answer to (c)(iii) to calculate the amount, in mol, of iodine that reacted with sodium sulfite when solution FA 5 was prepared. amount of $$\(\mathrm{I}_{2}\)$$ reacted with sodium sulfite $$\(=\)$$ .......................... $$\(\mathrm{mol}\)$$ (v) Use your answer to (c)(iv) to calculate the relative formula mass, $$\(M_{\mathrm{r}}\)$$, of hydrated sodium sulfite. $$\[ M_{\mathrm{r}} \text { of } \mathrm{Na}_{2} \mathrm{SO}_{3} \cdot \times \mathrm{H}_{2} \mathrm{O}= \]$$ .......................... (vi) Calculate the value of $$\(x\)$$. Show your working. $$\[ x= \]$$ .......................... Qualitative analysis For each test you should record all your observations in the spaces provided. Examples of observations include: - colour changes seen - the formation of any precipitate and its solubility (where appropriate) in an excess of the reagent added - the formation of any gas and its identification (where appropriate) by a suitable test. You should record clearly at what stage in a test an observation is made. Where no change is observed, you should write 'no change'. Where reagents are selected for use in a test, the name or correct formula of the element or compound must be given. If any solution is warmed, a boiling tube must be used. If a solid is heated, a hard-glass test-tube must be used. Rinse and reuse test-tubes and boiling tubes where possible. No additional tests should be attempted.

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