Question

The molality of 2% (W/W) $NaCl$ solution is nearly:
(a)- 0.02 m
(b)- 0.35 m
(c)- 0.25 m
(d)- 0.45 m

Answer 1

Hint: The molality of the solution can be calculated by dividing the number of moles of the solute by the mass of the solvent in kg. The 2% weight of the sodium chloride can be converted into grams and then the number of moles can be calculated.

Complete answer:
There are many ways for calculating the concentration of the solution and molality is one of them. The molality of the solution is calculated based on two factors i.e., moles of the solute and mass of the solvent by dividing the moles of the solute by the mass of the solvent.
In the question, it is given that 2% (W/W) $NaCl$ solution, so it means that the mass of the solute is in grams and mass of the solution is also in grams and 2% $NaCl$ solution is equal to 2 grams of $NaCl$ is present in 100 grams of the solution.
The solution is made up of solute and solvent, here solute is $NaCl$and the solvent is water. We know the value of solute and solution so we can calculate the value of the solvent.
So the mass of solvent will be:
$100-2=98\text{ g = 0}\text{.098 kg}$
Now we have to calculate the number of moles of $NaCl$, the given mass of $NaCl$ is 2 grams and the molecular mass of $NaCl$ is:
$23+35.5=58.5$
So the number of moles = $\dfrac{2}{58.5}=0.034$
Now we can calculate the molality of the solution as:
$\dfrac{0.034}{0.098}=0.35$
So the molality of the given solution is 0.35 m.

Therefore, the correct answer is option (b)- 0.35 m.

Note:
The mass of the solvent for calculating the molality of the solution must be in kg. The number of moles is calculated by dividing the given mass of the solute by the molecular mass of the solute.