Question

Weight of oxalic acid that will be required to prepare a $1000mL$ $\left( {\dfrac{N}{{20}}} \right)$ solution is:
A: $\dfrac{{126}}{{100}}g$
B: $\dfrac{{63}}{{40}}g$
C: $\dfrac{{63}}{{20}}g$
D: $\dfrac{{126}}{{20}}g$

Answer 1

Normality of a substance is defined as the number of gram equivalents present in $1000mL$of solution. Equivalent weight of a substance can be found by dividing molecular mass of the substance with valence.
Formula used: normality$= \dfrac{{m \times 1000}}{{E \times V}}$
Equivalent mass$= \dfrac{M}{{valency}}$
Where $m$ is given mass, $E$ is equivalent mass, $V$ is volume of solution and $M$ is molecular mass

Complete step by step answer:
We know normality of a substance is defined as the number of gram equivalents present per $1000mL$ of solution. Molecular formula of oxalic acid is ${C_2}{H_2}{O_4}$ . Formula to find normality is stated above but we have to find equivalent mass first. Formula to find equivalent mass is:
Equivalent mass$= \dfrac{M}{{valency}}$ where $M$ is molecular mass
Valency is the charge that a molecule will possess if the molecule is ionic. Molecular mass of the compound is found by adding mass of individual atoms. Molecular formula of a given compound that is oxalic acid is ${C_2}{H_2}{O_4}$. There are two atoms of carbon, two atoms of hydrogen and four atoms of oxygen. Atomic mass of carbon is$12$, hydrogen is $1$ and oxygen is $16$. So molecular formula of compound is:
$M = \left( {2 \times 12} \right) + \left( {2 \times 1} \right) + \left( {4 \times 16} \right)$
$M = 24 + 2 + 64$
Solving this we get,
$M = 90$
But there are two molecules of water associated with oxalic acid. Molecular mass of one molecule of water is $18$. Therefore two molecules of water will weigh $36$. Now the molecular mass of oxalic acid is$90 + 36 = 126$. Valency of this acid is two (as two ${H^ + }$ ions are released when dissolved). Applying above formula equivalent mass will be:
Equivalent mass$= \dfrac{{126}}{2} = 63$
Now, normality of solution$= \dfrac{1}{{20}}$ (given)
Volume of solution$\left( V \right) = 1000mL$ (given)
Equivalent mass$\left( E \right) = 63g$ (given)
Given mass$=$ ? (We have to find)
Using formula of normality that is,
Normality$= \dfrac{{m \times 1000}}{{E \times V}}$
Substituting all the values that are given and we have calculated (mass is unknown quantity) we get,
$\dfrac{1}{{20}} = \dfrac{{m \times 1000}}{{63 \times 1000}}$
Simplifying this equation we can calculate mass that is,
$m = \dfrac{{63}}{{20}}$
So the correct answer is option C that is $\dfrac{{63}}{{20}}g$.

Note:
Molarity and molality is also a way to represent concentration of solution. Molarity of a solution is defined as the number of moles of a substance that are dissolved in $1000mL$ of solution and molality is defined as number of moles dissolved in $1000Kg$ of solvent.