Question

The normality of orthophosphoric acid having purity of $70\%$ by weight and specific gravity $1.54$ is
A. $11N$
B. $22N$
C. $33N$
D. $44N$

Answer 1

The normality of a solution is defined as the weight of solute per litre of the solvent. Its unit can be represented as "N" (or) "$g/L$". The density of orthophosphoric acid can be calculated by multiplying specific gravity with a density of water.

Complete step by step answer:
The density of orthophosphoric acid can be calculated as,
$\rho (orthophosphoric \, acid) = specific\,gravity \times \rho (water)$
Where,
$\rho$-density of the respective compounds
It is known that the density of water is $997kg/{m^3}$. In terms of grams, it can be written as,
$\rho (water) = 0.997g/{m^3}$
$\Rightarrow \rho (orthophosphoric\,acid) = 1.54 \times 0.997g/{m^3}$
$\Rightarrow \rho (orthophosphoric\,acid) = 1.535g/{m^3}$
It is known that one millilitre is equal to one cubic metre. Thus, density of orthophosphoric acid in terms of grams per millilitre is $1.535g/ml$
Thus, the mass of orthophosphoric acid in terms of gram per litre i.e. gram in $1000ml$ can be written as
$mass = 1535g$
Thus, the weight of orthophosphoric acid can be calculated as,
$\Rightarrow 1535g \times \dfrac{{70}}{{100}} = mass$
Since orthophosphoric acid contains only $70\%$ by weight.
$\Rightarrow mass = 1074.5g$
Thus, the mass of orthophosphoric acid = $1074.5g$
The equivalent weight of orthophosphoric acid = $32.67g/mol$
The normality of orthophosphoric acid can be calculated as,
$Normality = \dfrac{{1074.5g \times 1000ml}}{{32.67g/mol \times 1000ml}}$
$\Rightarrow Normality = 32.89N \approx 33N$
The normality of orthophosphoric acid having a purity of $70\%$ by weight and specific gravity $1.54$ is $33N$ .

So, the correct answer is Option C.

Note: The important terms used to calculate the concentration of the solution are Normality, Molality, Molarity, Mole fraction, and Parts per million. The molecular formula for orthophosphoric acid is ${H_3}P{O_4}$. It is also known as phosphoric acid. The pure phosphoric acid is a colourless solid. Phosphoric acid is a corrosive chemical and when contacting, it may lead to eye irritation, burns, etc. It is used in the synthesis of albumin derivatives and also it is used in dental cement.