Question

12 g of an impure sample of arsenious oxide was dissolved in water containing 7.5 g of sodium bicarbonate and the resulting solution was diluted by 250 ml. 25 ml of this solution was completely oxidized by 22.4 ml of a solution of iodine. 25 ml of this iodine solution reacted with same volume of a solution containing 24.8 g of sodium thiosulphate solution ($N{{a}_{2}}{{S}_{2}}{{O}_{3}}.5{{H}_{2}}O$) in one litre. Calculate the percentage of arsenious oxide in the sample. (Atomic mass of As = 75)

Answer 1

Hint: Normality is generally used to measure the concentration of a solution. It is represented by the letter N and sometimes referred to as the equivalent concentration of a solution. Units of normality expressed as $eq{{L}^{-1}}$ or $meq{{L}^{-1}}$.

Complete Solution :
Normality can also be defined as the number of gram or mole equivalents of solute present in one litre of a solution. Where equivalent represents the number of moles of reactive units in a compound.
Normality is calculated by the formula: $N=\dfrac{Weight~of~solute}{equivalent~weight\times volume}$
Normality of $N{{a}_{2}}{{S}_{2}}{{O}_{3}}.5{{H}_{2}}O$ solution = $\dfrac{24.8}{248\times 1}=0.1N$

Applying the normality equation:
${{N}_{1}}{{V}_{1}}={{N}_{2}}{{V}_{2}}$
$Volume~of~A{{S}_{2}}{{O}_{3}}solution~in~NaHC{{O}_{3}}\times Normality$ = $Volume~of~iodine~solution\times Normality$

- We have to find out normality of $A{{S}_{2}}{{O}_{3}} solution in NaHC{{O}_{3}}$, let us suppose it as N
$25\times N = 22.4\times 0.1$
$N=\dfrac{22.4\times 0.1}{25} = 0.0896$
Amount of $A{{s}_{2}}{{O}_{3}}$ present in 250 ml of solution is given by:
$N\times \dfrac{Equivalent~mass~of~A{{s}_{2}}{{O}_{3}}}{1000}\times 250$
$0.0896\times \dfrac{198}{4\times 1000}\times 250 = 1.1088g$
Percentage of $A{{s}_{2}}{{O}_{3}}$ = $\dfrac{1.1088}{12}\times 100=9.24$
Percentage of arsenious oxide in the sample is 9.24%.

Note: Arsenious oxide is an inorganic compound. It is generally used as a medication to treat a type of cancer called acute promyelocytic leukemia in which it is by injection directly to the vein. It is also used in the manufacture of wood preservatives, pesticides etc.