Question

The weight of ${{H}_{2}}{{C}_{2}}{{O}_{4}}.2{{H}_{2}}O$required to prepare $500ml$of $0.2N$solution is
A.$126g$ [EAMCET$1991$]
B.$12.6g$
C.$63g$
D.$6.3g$

Answer 1

Hint: As atoms, molecules are very small particles hence the mole concept is very useful in chemistry. Here we also need this concept to calculate equivalent weight and normality. First, calculate the equivalent weight of ${{H}_{2}}{{C}_{2}}{{O}_{4}}.2{{H}_{2}}O$and then put the value in the respective equation of normality that is given in below.

Formula Used:(i)Equivalent weight$=\dfrac{M}{Acidity}$
Here $M=$Molar mass
(ii)Normality,$N$$=\dfrac{W}{Eq.weight}\times \dfrac{1000}{V(ml)}$
$V=$The volume of solution in $ml$.

Complete answer:In this problem, we have a compound ${{H}_{2}}{{C}_{2}}{{O}_{4}}.2{{H}_{2}}O$ , which is a chemical formula of oxalic acid.
As we know the atomic weight of Hydrogen($H$)$=1$
The atomic weight of carbon($C$)$=12$
And the atomic weight of oxygen($O$)$=16$
Therefore the molar mass of oxalic acid${{H}_{2}}{{C}_{2}}{{O}_{4}}.2{{H}_{2}}O$is given by
${{M}_{{{H}_{2}}{{C}_{2}}{{O}_{4}}.2{{H}_{2}}O}}=$ ($6\times $atomic weight of hydrogen)$+$($2\times $atomic weight of carbon)$+$($6\times $atomic weight of oxygen)
$\therefore {{M}_{{{H}_{2}}{{C}_{2}}{{O}_{4}}.2{{H}_{2}}O}}=(6\times 1)+(2\times 12)+(6\times 16)=126gm/mol$
Here acidity is the number of replaceable ${{H}^{+}}$ions. As oxalic acid is a dibasic acid and it has two replaceable ${{H}^{+}}$ions per mole.
Now the equivalent weight of oxalic acid $=\dfrac{{{M}_{{{H}_{2}}{{C}_{2}}{{O}_{4}}.2{{H}_{2}}O}}}{Acidity}=\dfrac{126}{2}=63g$
Given the normality$=0.2N$, and volume of the solution,$V=500ml$
The required weight of oxalic acid can be calculated by using the following formula,
 Normality,$N$$=\dfrac{W}{Eq.weight}\times \dfrac{1000}{V(ml)}$
Or,$W=\dfrac{Eq.weight\times N\times V(ml)}{1000}$
Putting these values in this equation we get,
Weight of oxalic acid,$W=\dfrac{63\times 0.2\times 500}{1000}=6.3g$

 Thus, option (D) is correct.

Note: Molarity is the molar concentration of a solution that the number of moles of any chemical compound is dissolved per $litre$ of the solution. Molality is the number of moles that are dissolved per $kg$ solvent. The temperature does not affect molality but molarity is affected by temperature. With increasing temperature molarity decreases.