Question

What is the molarity of a solution that contains 20 g of NaOH in 1 L?

• A. 2 M
• B. 0.5 M
• C. 0.02 M
• D. 0.005 M

Answer 1

The correct option is B

0.5 M

Mass of NaOH taken = 20 g
Volume of the aqueous solution = 1 L
Gram molecular mass of NaOH = 23 + 16 + 1 = 40 g
We know that, Molarity of a solution
= $\frac{\text{Number of moles of solute}}{\text{Volume of the solution (litres)}}$
= $\frac{\text{Given mass (gram)}}{\text{Gram molecular mass}}\times \frac{1}{\text{Volume of solution (litre)}}$

Putting the values of given mass of NaOH, its gram molecular mass in equation and volume of the solution in the above equation, we get
Molarity of a solution = $\frac{20}{40}\times \frac{1}{1}$ = 0.5 M
Hence, the molarity of the given solution is 0.5 M.