Question

The mass of oxalic acid (hydrated) required to prepare $500mL$ of its $0.1N$ solution is:
A.$0.315g$
B.$6.3g$
C.$3.15g$
D.$63.0g$

Answer 1

Hint: Normality is used to express concentration of solutions. It is defined as the number of gram equivalents of solute present in one litre of the solution. Normality is a very useful concept and it has a wide range of applications.

Complete step by step answer:
The formula to calculate normality is,
Normality, N $ = \dfrac{{{n_{eq}}}}{V}$
Where ${n_{eq}}$ is the number of gram equivalents of the solute and V is the volume of solution in litre.
Number of gram equivalents can be calculated by the formula,
${n_{eq}} = \dfrac{{{w_B}}}{E}$
Where ${w_B}$ is the weight of solute in grams and E is the equivalent weight of the solute. Equivalent weight of an element is the mass of one equivalent of the element. It is the mass of the given compound that will combine or displace a fixed quantity of another compound. For acids, it can be calculated by the following formula,
$E = \dfrac{M}{x}$
Where M is the molar mass and x is the number of replaceable hydrogens of the acid. So the equation for normality can be written as,
$N = \dfrac{{{w_B}}}{{E \times V}}$
Given,
$N = 0.1N$
$V = 500mL = 0.5L$
We need to calculate ${w_B}$.
First we need to calculate the equivalent weight of oxalic acid. Formula of hydrated oxalic acid is ${(COOH)_2}.2{H_2}O$. Molar mass is equal to $126g/mol$. The number of replaceable hydrogens on oxalic acid is two. Hence the equivalent weight is,
$E = \dfrac{M}{x} = \dfrac{{126}}{2} = 63$
Rearranging the equation for normality, we get the equation to find out weight of oxalic acid as,
${w_B} = N \times E \times V$
Let us substitute the values.
${w_B} = 0.1 \times 63 \times 0.5 = 3.15g$
Hence the mass of oxalic acid (hydrated) required to prepare $500mL$ of its $0.1N$ solution is $3.15g$. Option C is correct.

Note:
Equivalent weight of an acid can be equal to its molecular weight when the number of replaceable hydrogen is equal to one. Acids like HCl and $HN{O_3}$ have only one hydrogen to donate. i.e. they are monobasic acids. For monobasic acids equivalent weight and molecular weight are equal.