Question

How can I calculate the molality of pure water?

Answer 1

Step 1: Calculate the mass of water:

In the case of pure water, the density is = $0.9970749\mathrm{kg}{\mathrm{lt}}^{-1}$

Therefore in 1lt volume of water, the mass will be calculated as;

$$\begin{gathered} =\mathrm{density}\times \mathrm{volume} \\ =0.9970749\mathrm{kg}/\mathrm{lt}\times 1\mathrm{lt} \\ =0.9970749\mathrm{kg} \end{gathered}$$

Step 2: calculate no of moles for pure water:

The molecular mass of pure water is given as= $18.0148\mathrm{g}{\mathrm{mol}}^{-1}=0.0180148\mathrm{kg}{\mathrm{mol}}^{-1}$

No of moles of pure water can be calculated as = $\frac{\mathrm{molecular}\mathrm{mass}\mathrm{of}\mathrm{water}}{\mathrm{mass}\mathrm{of}\mathrm{pure}\mathrm{water}\left(\mathrm{Kg}\right)}$

=$\frac{0.9970749\mathrm{kg}}{0.0180148\mathrm{kg}{\mathrm{mol}}^{-1}}$

= $55.348$

Step 3: Formula given for molality:

Molality is termed as the number of moles of solute dissolved per kilogram of solvent.

Molality is represented by ‘m’.

Formula of molality: :$\mathrm{m}=\frac{\mathrm{Moles}\mathrm{of}\mathrm{the}\mathrm{solute}}{\mathrm{mass}\mathrm{of}\mathrm{the}\mathrm{solvent}\left(\mathrm{Kg}\right)}$

Step 4 : Calculating the molality of pure water:

So, the molality of pure water will be (M) :$\frac{55.348}{0.9970749}=55.510\text{m}$

Hence, the molality of pure water can be calculated by using the above mentioned formula which is found out to be $55.510\mathrm{m}.$