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## Homework Statement

:[/B]The percentage of copper in a Copper (II) salt can be determined by using a thiosulphate titration. 0.305g of a copper (II) salt was dissolved in water and added to an excess of KI solution liberating Iodine. The liberated Iodine required 24.5 ##dm^3## of a 0.1 mole ##dm^{-3}## solution of sodium this sulphate. The percentage of copper, by mass, in the copper (II) salt is:

1. 64.2

2. 51.0

3. 48.4

4. 25.5

## Homework Equations

:[/B]## The Attempt at a Solution

:[/B]0.1 mole per ##dm^3## means 0.1 mole per litre.

Valence factor for sodium thiosulphate is 2.

So, there are 0.1×2=0.2 eqs of thiosulphate in 1 litre, which basically means the normality is 0.2.

No. of eqs of ##Cu^{2+}## = No. of eqs of Iodine liberated = No. of eqs of thiosulphate used = ##\frac {24.5}{1000} ×0.2 = 4.9 ×10^{-3}##.

So, weight of ##Cu^{2+}## ions = ##4.9 ×10^{-3} ×63.5=0.31115g##, which is greater than the weight of the sample.

Where am I going wrong?