Question

A 2M 1 litre solution of Copper Sulphate is titrated with a 2M solution of KI. The amount of KI solution required was 2 litres. What is the amount of sodium thiosulphate required to back titrate the iodine formed to iodide?

Answer 1

Hint: Redox titration using sodium thiosulphate, $N{{a}_{2}}{{S}_{2}}{{O}_{3}}$ (usually) as a reducing agent is known as iodometric titration since it is used specifically to titrate iodine. The molar mass of sodium thiosulphate is 158.11 g/mol.

Complete step by step answer: Direct titration is a way to determine the contents of a substance quantitatively. Scientists may be aware of a reactant, but not know the reactant’s quantity. Direct titration is sometimes based on indicators that respond to the analyzed material, called the analyte
A back titration is a titration method where the concentration of an analyte is determined by reacting it with a known amount of excess reagent. The remaining excess reagent is then titrated with another, second reagent. The second titration's result shows how much of the excess reagent was used in the first titration, thus allowing the original analyte's concentration to be calculated. A back titration may also be called an indirect titration
In the iodometric titration, the substance to be estimated (such as ozone) reacts with iodide ion to liberate iodine.
\[{{O}_{3}}+2{{I}^{-}}\to {{I}_{2}}+{{O}_{2}}+{{O}^{2-}}\]
The liberated iodine is estimated with sodium thiosulphate solution.
\[{{I}_{2}}+2{{S}_{2}}{{O}_{3}}^{2-}\to 2{{I}^{-}}+{{S}_{4}}{{O}_{6}}{{^{2}}^{-}}\]
The titration of $C{{u}^{2+}}~$ with ${{I}^{-}}$:
\[\begin{align}
& 2C{{u}^{2+}}+4{{I}^{-}}\to 2CuI(s)+{{I}_{2}} \\
& 2N{{a}_{2}}{{S}_{2}}{{O}_{3}}+{{I}_{2}}\to {{S}_{4}}{{O}_{6}}^{2-}+2{{I}^{-}} \\
\end{align}\]

Therefore, from the above equation, 2 moles of sodium thiosulphate needed to titrate iodine evolved, hence 316 gram of sodium thiosulphate will be needed.

Note: The iodometric titration is a general method to determine the concentration of an oxidising agent in solution. In an iodometric titration, a starch solution is used as an indicator since it can absorb the ${{I}_{2}}$ that is released. This absorption will cause the solution to change its colour from deep blue to light yellow when titrated with standardised thiosulfate solution. This indicates the end point of the titration.