Question

The mass of oxalic acid crystals (${{H}_{2}}{{C}_{2}}{{O}_{4}}.2{{H}_{2}}O$) required to prepare $50 mL$ of a $0.2 N$ solution is:
(a)- 4.5 g
(b)- 6.3 g
(c)- 0.63 g
(d)- 0.45 g

Answer 1

Hint: The amount of oxalic acid can be calculated by multiplying the molecular weight of the oxalic acid to the molarity of the solution and volume of the solution in liters. The molarity of the acid can be calculated by dividing the normality of the solution by the basicity.

Complete step by step solution:
The amount of oxalic acid can be calculated by multiplying the molecular weight of the oxalic acid to the molarity of the solution and volume of the solution in liters.
Given the formula of oxalic acid crystals is ${{H}_{2}}{{C}_{2}}{{O}_{4}}.2{{H}_{2}}O$ so, the molecular mass of the oxalic acid is:
$=2(1)+2(12)+4(16)+2(18)$
$=2+24+64+36$
$=126\text{ g/mol}$
So the molecular mass of oxalic acid is 126 g/mol.
The molarity of the acid can be calculated by dividing the normality of the solution by the basicity.
The basicity of the oxalic acid is 2 and the normality of the solution is 0.2 given in the question. So the molarity of the solution will be:
$Molarity=\dfrac{0.2}{2}=0.1\text{ M}$
Molarity is 0.1.
Given the volume of the solution is 50 mL, so in liters, it will be 0.05 L.
Now we have all the three factors, so we can calculate the amount of oxalic as:
$\implies 126\text{ x 0}\text{.1 x 0}\text{.05}$
$\therefore 0.63\text{ g}$
So the amount of oxalic acid required is 0.63 g.

Therefore, the correct answer is an option (C)- 0.63 g.

Note: Basicity of the compound is equal to the number of hydrogen ions that can be displaced in the solution, so the oxalic acid has 2 displaceable hydrogen ions. Therefore, its basicity is 2.