Question

0.16g of dibasic acid requires 25 mL of decinormal NaOH solution for complete neutralisation. The molecular weight of acid will be:
A. 32
B. 64
C. 128
D. 256

Answer 1

Hint: The normality of solution is defined as the gram of the equivalent solute which is dissolved in the cubic decimeter or the 1 L of the solution. The strength of the given solution can be found by just multiplying the normality and the equivalent weight of the substance. Normality is expressed by N. this unit is preferred during titration.

Complete step by step answer:
To approach this question we need to find out the gram equivalent of the substance used here which is $NaOH$. So it is given in question that $25 mL$ of decinormal sodium hydroxide solution has been used. So the value of a decinormal solution means $0.1$. So now let us calculate the gram equivalent
Gram equivalent NaOH = $$\dfrac{{25}}{{1000}} \times 0.1 = 0.0025$$ moles
As the question says that the acid is dibasic that means for every mole of the acid used we need 2 moles of base.
So moles of acid = $$\dfrac{{0.0025}}{2} = 0.00125$$ moles
The formula to calculate molar mass using given mass and moles is the following:
Molar Mass = $$\dfrac{{{\text{mass given}}}}{{{\text{moles}}}}$$
Now the mass given is $0.16$ and moles is $0.00125$
Substituting the values in formula we get,
Molar mass = $$\dfrac{{{\text{0}}{\text{.16}}}}{{{\text{0}}{\text{.00125}}}}{\text{ = 128gmo}}{{\text{l}}^{{\text{ - 1}}}}$$

So, the correct answer is Option (C).

Note: The molarity of the solution is defined as the moles of solute present in the one litre of the solution. It is expressed as M. the relation between normality and molarity is given below:
$${{\text{normality} \times \text{equivalent mass} = \text{molarity} \times \text{molar mass}}}$$