Molality

Molality is also known as molal concentration. It is a measure of solute concentration in a solution. The solution is composed of two components; solute and solvent. There are many different ways to express the concentration of the solutions like molarity, molality, normality, formality, volume percentage, weight percentage and part per million.

Molality Definition

Molality is defined as the “total moles of a solute contains in a kilogram of a solvent.” The term needs to calculate the mass of solvent and moles of solute.

Molality FormulaMolality (M) = \(\frac {Number of moles of solute}{Mass of solvent in kgs}\)

Molality (Mi) = \(\frac{g \times 1000}{W \times m}\)

Difference Between Molarity and Molality

Molality Molarity
Mass of the solvent Volume of the whole solution
Unit sign expressed as (m) Unit sign expressed as (M)
It has units of moles / kg It has units of moles / liter

Molality Examples

Question: Calculate the molality of a solution where 0.5 grams of toluene \((C_{7}H_{8})\) is dissolved in 225 grams of Benzene \((C_{6}H_{6})\).

Solution

Calculate the moles of given solute.

Toluene – Molecular weight = \(C_{7}H_{8} = 7 \times 12 \times + 1 \times 8 = 92 moles / gram\)

Using the formula:

Moles of Toluene = \(\frac{Mass\; in \;grams}{Molecular\; weight}\) = 0.054 mole.

So, the mole of toluene is 0.054 mole.

Now calculate the kilogram of solvent.

\(\frac{225\; grams\; of \;Benzene}{1000}\) = 0.225 kilogram

As the final step, calculate the molality using the formula.

Molality (m) = \(\frac{Moles\;of\;Toluene}{Mass\;of\;Benzene\;in\;grams}\)

Molality (m) = \(\frac{0.054 moles}{0.225 kg}\)

Molality = 0.24 m


Practise This Question

A small mass m is attached to a massless string whose other end is fixed at p as shown in the figure. The mass is undergoing circular motion in the x-y plane with centre at 0 and constant angular speed ω. If the angular momentum of the system, calculated about o and p are denoted by Lo and Lp respectively, then