Question

If 5.85 g of NaCl (molecular weight = 58.5) is dissolved in water and the solution is made up to 0.5 litre, the molarity of the solution will be:
(a) 0.2
(b) 0.4
(c) 1.0
(d) 0.1

Answer 1

Hint: Firstly, calculate the number of moles by dividing weight of the compound by its molecular weight. Then, use this to calculate the molarity of the solution in moles per litre volume.

Complete step by step answer:
Molarity, often represented by ‘M’, is a term used to express concentration in terms of moles per litre of a solution.
Therefore, we define it as - “Molarity of any solution is number of moles of solute per litre of solution”. It has the following formula –
\[\text{Molarity=}\dfrac{\text{moles of solute (mol)}}{\text{volume of solution (L)}}\]
From the above formula, we can see that the SI unit of Molarity is mol/L.
According to the question,
weight of NaCl = 5.85 g
molecular weight of NaCl= 58.5
From this, we can calculate the number of moles of NaCl
= \[\dfrac{\text{weight of NaCl}}{\text{molecular weight of NaCl}}\]
= \[\dfrac{5.85}{58.5}\] = 0.1 moles
Now, let us calculate the Molarity of solution –
Molarity = \[\dfrac{\text{moles of solute (mol)}}{\text{volume of solution (L)}}=\dfrac{0.1}{0.5}\]mol/L = 0.2 mol/L
Therefore, the answer is – option (a) – the molarity of the solution is 0.2 mol/L.

Additional Information:
The ‘Concentration’ of a solution is defined as the relative amount of solute present in a solution.

Note: People often confuse between Molarity and Molality. Molality is represented by ‘m’, whereas molarity is represented by ‘M’. Molality of any solution can be defined as the number of moles of solute present per kg of solvent. It can be represented by the following –
 \[\text{Molality=}\dfrac{\text{moles of solute (mol)}}{\text{weight of solvent (kg)}}\]
Its SI unit is mol/kg.