Question

316 g of sodium thiosulphate was dissolved in water and made up of 250 mL. What is the normality of the solution?

Answer 1

Hint: The normality of the solution is obtained from gram equivalent weight of the solute present in the one litre of solution. It is a concentration term.

Complete step by step answer:
Given the mass of sodium thiosulphate solute, ${{w}_{A}}$ as 316 g which is dissolved in water to form the solution of volume 250 mL. Then, the concentration of the solution formed expressed in terms of normality will be as follows:
$Normality=\dfrac{Number\,of\,gram\,equivalents\,of\,solute}{Volume\,of\,solution\,\,in\,L}$ -------- (a)
The number of gram equivalent is obtained from the relation:
$Number\,of\,gram\,equivalent=\dfrac{weight\,of\,solute}{equivalent\,mass}=\dfrac{{{w}_{A}}}{Eq}$ and further the equivalent mass is obtained as $Eq=\dfrac{Molar\,mass}{n-factor}=\dfrac{{{M}_{A}}}{n}$.
So, the molar mass (${{M}_{A}}$ ) of sodium thiosulphate pentahydrate, $N{{a}_{2}}{{S}_{2}}{{O}_{3}}.5{{H}_{2}}O$ is $=(2\times 23)+(2\times 32)+(3\times 16)+5(2\times 1+16)=248\,g$and the n-factor is 1, as the compound dissociates into its ions on dissolving in water as:
$N{{a}_{2}}{{S}_{2}}{{O}_{3}}(s)+{{H}_{2}}O(l)\to 2N{{a}^{+}}+{{S}_{2}}O_{3}^{2-}$
Then substituting the values of weight, molar mass, n-factor, and volume of solution in equation (a), we get,
$Normality,\,N=\dfrac{{{w}_{A}}\times n}{{{M}_{A}}\times V}$
$\begin{align}
& N=\dfrac{316\times 1}{248\times 250\times ({{10}^{-3}}L)}=5.09\,N \\
& \\
\end{align}$

Therefore, the normality of the solution formed by dissolving 316 g of sodium thiosulphate in water is 5.09N.

Additional information: Often the normality is used to measure the reactive capacity of the molecule, for the equivalence factor (n) is considered. Like in the acid-base reaction, involving the proton or hydroxide ion concentration in solution or in the redox reaction, the number of electrons gained or lost during reaction is considered in the normality.

Note: The normality is used to express the concentration of the solution formed. Also, known as the equivalent concentration as we use the number of gram equivalents instead of the moles of the solute as taken in other concentration terms like molarity and molality.