Ncert Exercises & Solutions

The molality of a solution containing 18.25 g of HCl gas in 500 g of water is -

1.	0.1 m	2.	1 M
3.	0.5 m	4.	1 m

Hint: Use the formula of molality, that is, \(\frac{number \ of \ moles \ of \ solute}{amount \ of \ solvent (kg)}\)

Molality is defined as the number of moles of solute present in 1 kg of solvent. It is denoted by m.

Thus,

Molality (m) = \frac{Moles of solute}{Mass of solvent (in kg)}

Given that,

Mass of solvent (H_{2} O) = 500 g = 0.5 kg

Weight of HCl = 18.25 g

Molecular weight of HCl = 1 \times 1 + 1 \times 35.5 = 36.5

Moles of HCl = \frac{18.25}{36.5} = 0.5

m = 0.5/0.5 = 1

Hence, molality value is 1m