MediumChemistryHigh School (11–12)The d- and f-Block Elements › Transition Elements (d-Block)Numerical

A student weighs 0.1470 g of pure potassium dichromate, with a molar mass of 294 g/mol…

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A student weighs $0.1470\ \text{g}$ of pure potassium dichromate, with a molar mass of $294\ \text{g/mol}$ . It is dissolved, and acid and excess potassium iodide are added: $\text{Cr}_2\text{O}_7^{2-} + 6\text{I}^- + 14\text{H}^+ \rightarrow 2\text{Cr}^{3+} + 3\text{I}_2 + 7\text{H}_2\text{O}$ . The iodine that forms needs $29.4\ \text{mL}$ of sodium thiosulfate solution, by $\text{I}_2 + 2\text{S}_2\text{O}_3^{2-} \rightarrow 2\text{I}^- + \text{S}_4\text{O}_6^{2-}$ . Find the amount of dichromate, the amount of iodine and the concentration of the thiosulfate solution.

Need a reminder? The key ideaShow

Moles = mass in g divided by molar mass in g/mol, or concentration in mol/L times volume in litres. Take every mole ratio from the balanced equation, and follow the chain one step at a time.

The worked solution

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Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.

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