Single Choice

Molarity of $$4\%$$ (w/v) solution of $$NaOH$$ is:

A$$0.1$$
B$$0.5$$
C$$0.001$$
D$$1.0$$
Correct Answer

Solution

Hint: Molarity (M) is the amount of a substance in a certain volume of solution.

Mathematically, it can be written as-
$$M=$$ number of moles of solute $$(n) \div$$ volume of solution $$(L)$$

Step 1: Find moles of $$NaOH$$
A per cent $$w/v$$ solution is calculated with the following formula using the gram as the base measure of weight (w):
$$% w/v$$ = g of solute/$$100$$ mL of solution
Weight = $$4$$gm

$$\text { Moles }=\frac{4\mathrm{~g}}{40 \mathrm{~g} / \mathrm{mol}}$$

Step 2: Find Molarity
As we know,
$$M=$$ number of moles of solute $$(n) \div$$ volume of solution $$(L)$$

The molecular mass of $$\mathrm{NaOH}$$ is $$=40$$

So, Molarity $$=\frac{4}{40} \times \frac{1000}{100}=1 \mathrm{M}$$

So, The correct option is $$D$$.

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