Question

Hydroxylamine reduces iron (III) according to the following reaction:
\[2N{{H}_{4}}OH+4F{{e}^{3+}}\to {{N}_{2}}{{O}_{(g)}}\uparrow +{{H}_{2}}O+4F{{e}^{2+}}+4{{H}^{+}}\]
Iron (II) thus produced is estimated by titration with a standard permanganate solution. The reaction is:
\[MnO_{4}^{-}+5F{{e}^{2+}}+8{{H}^{+}}\to M{{n}^{2+}}+5F{{e}^{3+}}+4{{H}_{2}}O\]
A 10ml sample of hydroxylamine solution was diluted to 1L. When 50ml of this diluted solution was boiled with an excess of iron (III) solution, the resulting solution required 12ml of 0.02M $KMn{{O}_{4}}$ solution for complete oxidation of iron (II). Calculate the weight of hydroxylamine in 1L of the original solution (H=1, N=14, O=16, K=39, Mn=55, Fe=56):
(A) $40g{{L}^{-1}}$
(B) $80g{{L}^{-1}}$
(C) $22g{{L}^{-1}}$
(D) $62g{{L}^{-1}}$

Answer 1

Hint: Start by writing, the relation between molarity and normality of $KMn{{O}_{4}}$. Then find the relation between volume of $KMn{{O}_{4}}$ and ferrous solution and correspondingly hydroxylamine solution required for that much amount of ferrous solution. Proceed by taking into account equivalent weight of hydroxylamine.

Complete step by step answer:
-0.02M $KMn{{O}_{4}}$ is equivalent to 0.01N $KMn{{O}_{4}}$
\[\text{12ml}\,\text{0}\text{.1N}\,\text{KMn}{{\text{O}}_{\text{4}}}\equiv 12ml\,\text{0}\text{.1N}\,\text{F}{{\text{e}}^{\text{2+}}}\equiv 12ml\,\text{0}\text{.1N}\,\text{Hydroxylamine}\]
10ml of original hydroxylamine solution is diluted to 1L to make 0.1N hydroxylamine solution. So, now we need to know how much mass of hydroxylamine is present in a 50ml diluted solution.
- Equivalent weight of hydroxylamine $=\dfrac{Mol.\,mass}{2}=\dfrac{33}{2}=16.5g$
So, mass of hydroxylamine in 12ml of 0.1N hydroxylamine$=\dfrac{N\times V\times E}{1000}=\dfrac{0.1\times 12\times 16.5}{1000}=0.0198g\approx 0.02g$
Therefore, 50ml diluted solution of 0.1N hydroxylamine contains 0.02g.
So, one litre diluted solution of 0.1N hydroxylamine contains $\dfrac{0.02}{50}\times 1000=0.4g$

Therefore, 10ml original solution contains 0.4g.
So, 1000ml original hydroxylamine solution will contain 40g.
So, the correct answer is “Option A”.

Note: Don’t get confused with normality and molarity. While calculating mass for quantitative analysis, always take normality into consideration to make calculation simpler.