Question

To make a saturated solution, 36 grams of sodium chloride is dissolved in hundred grams of water at 293 K. Find the concentration at this temperature.

Answer 1

The molality of a solution is defined as the number of moles of solute in a solution divided by the weight of solvent in kilograms. Molarity is a concentration term. The concentration of the solution can also be defined in terms of weight by weight of the solution.

Complete Solution :
Molality of a solution is one way of expressing the concentration of the solute present in it. It is defined as the number of moles of solute per kilogram of solvent. It is defined by m. In mathematical terms, it is expressed as:
Molality (m) = $\dfrac{{Moles\,of\,solute}}{{Weight\,of\,solvent\,\left( {in\,kgs} \right)}}$
In the given question,
Molar mass of NaCl = 23 + 35.5 = 58.5 g
Moles of NaCl = $\dfrac{{36}}{{58.5}}$ = 0.615 moles
Weight of solvent = 100 g = 0.1 kg
Now,
Molality (m) = $\dfrac{{0.615}}{{0.1}}$
= 6.15 m
Also,
Mass of solute = 36 g
Mass of solvent = 100 g
Mass of solution = 36 g + 100 g = 136 g
Concentration of solution in $\% \dfrac{w}{w}$ = $\dfrac{{36}}{{136}} \times 100\%$
= 26.47 %
Hence, the concentration of solution is 6.15 m or 26.47 % .

Note: Note that saturated solution is the one which contains the maximum amount of solute which is capable of being dissolved or can also be defined as the solution in which the equilibrium point is reached and hence no more solute can be dissolved.