Question

Consider the titration of potassium dichromate solution with acidified Mohr's salt solution using diphenylamine as an indicator. The number of moles of Mohr's salt required per mole of dichromate is:
(A) 3
(B) 4
(C) 5
(D) 6

Answer 1

Hint: For this problem, we have to first write the reaction of the potassium dichromate solution with the Mohr's salt. Then we can easily observe the number of moles of Mohr's salt required for each potassium dichromate molecule.

Complete step by step solution:
-In the given question, we have to calculate the number of moles of Mohr's salt required for each molecule of dichromate.
-As we know that titration is the quantitative method to analyze the concentration of the unknown solution from the concentration of a known solution.
-In the titration of potassium dichromate with acidified Mohr's salt, the formation of potassium sulphate and chromium sulphate take place.
-In this reaction, the oxidation of ferrous sulphate takes place by the potassium dichromate because there is an increase in the oxidation state of iron from +2 to +3.
In $\text{FeS}{{\text{O}}_{4}}$,
$\text{x - 2 = 0}$; $\text{x = +2}$
In $\text{F}{{\text{e}}_{2}}{{(\text{S}{{\text{O}}_{4}}\text{)}}_{3}}$,
$\text{2x + (3 }\times \text{ -2) = 0}$; $\text{x = +3}$
-The balanced chemical reaction is shown below:
${{\text{K}}_{2}}\text{C}{{\text{r}}_{2}}{{\text{O}}_{7}}\ \text{+ 6FeS}{{\text{O}}_{4}}\,\text{+ 7}{{\text{H}}_{2}}\text{S}{{\text{O}}_{4}}\text{ }\to \text{ }{{\text{K}}_{2}}\text{S}{{\text{O}}_{4}}\ \text{+ C}{{\text{r}}_{2}}{{\text{(S}{{\text{O}}_{4}}\text{)}}_{3}}\text{ + 3F}{{\text{e}}_{2}}{{(\text{S}{{\text{O}}_{4}}\text{)}}_{3}}\,\text{+ 7}{{\text{H}}_{2}}\text{O}$
-Here we can see that on the reactant side the total number of molecules of Mohr's salt i.e. iron sulphate is 6 whereas only one molecule of potassium dichromate is consumed.
-So, we can say that for each molecule of potassium dichromate a total of 6 molecules of iron sulphate will be required.

Therefore, option (D) is the correct answer.

Note: In the given question, Mohr's salt is the mixture which is formed by the ferrous sulphate and ammonium sulphate and have a molecule formula of $\text{FeS}{{\text{O}}_{4}}{{\text{(N}{{\text{H}}_{4}}\text{)}}_{2}}\text{S}{{\text{O}}_{4}}$.