Question

What is molality of a solution prepared by dissolving 3 moles of solute in 1 kg of solvent?

Answer 1

Hint: Molality is defined as the number of moles of the solute per kilogram of the solvent. It is denoted by “m” and is expressed as: \[Molality{\text{ }}\left( m \right){\text{ }} = {\text{ }}\dfrac{{Moles{\text{ }}of{\text{ }}solute}}{{Mass{\text{ }}of{\text{ }}solvent{\text{ }}in{\text{ }}kg}}\]


Complete step by step answer:
Molality is one of the many different ways to describe the concentration of solution quantitatively. In order to understand what is molality, we have to understand what is a solution and what are the different types of ways to define the concentration of solution.
Solutions are homogeneous mixtures of two or more components. By homogenous mixture we mean that its composition is uniform throughout the mixture and properties too i.e. at every place in that solution we can find the same composition. Generally, the component that is present in the largest quantity in a solution is known as solvent and the one which is less is called solute. Solute can be one or more in number in a solution.
Since we know that molality is one of the methods to find the concentration of a solution. There are other methods which can be used to define the concentration of a solution. Let’s check out some of them so that we can differentiate them easily.
- Mass percentage: The mass percentage of a component of a solution can be defined as the ratio of mass of the component of solution to the total mass of the solution. It can be expressed as : Mass % of a component = (\[\dfrac{{Mass{\text{ }}of{\text{ }}the{\text{ }}component{\text{ }}in{\text{ }}the{\text{ }}solution}}{{Total{\text{ }}mass{\text{ }}of{\text{ }}the{\text{ }}solution}}\])x 100
- Molarity: Molarity is defined as the number of moles of solute dissolved in 1 liter of solution. It is denoted by “M” and can be expressed as
\[M{\text{ }} = {\text{ }}\dfrac{{Moles{\text{ }}of{\text{ }}solute}}{{Volume{\text{ }}of{\text{ }}solution\left( l \right)}}\]
Now, let’s apply the formula of molality to get out answer:
\[Molality{\text{ }}\left( m \right){\text{ }} = {\text{ }}\dfrac{{Moles{\text{ }}of{\text{ }}solute}}{{Mass{\text{ }}of{\text{ }}solvent{\text{ }}\left( {kg} \right)}}\]
By putting the values of moles of solute and mass of solvent, we get
 Molality (m) = $\dfrac{3}{1}$= 3

Therefore, the molality of a solution prepared by dissolving 3 moles of solute in 1 kg of solvent is 3.


Note: Most of the people get confused between molality and molarity due to their almost similar pronunciation but remember that molality is the number of moles of the solute per kilogram of the solvent whereas molarity is number of moles of solute dissolved in 1 liter of solution.