Question

How can I calculate the molality of pure water?

Answer 1

The molality of the solution is one of the major factors in calculating the concentration of the solution. The factors which can be used to calculate the molality of the solution are the number of moles of the solute and the mass of the solvent in kg. With the help of the density of pure water, the number of moles of the water can be calculated.

Complete step by step answer:
The concentration of the solution tells the strength of the solution. There are many factors that can be used for calculating the strength of the solution like molarity, molality, normality, mole fraction, ppm, etc. So, the molality of the solution is one of the major factors in calculating the concentration of the solution. There are two factors that are used for the calculation of the molality of the solution, i.e., the number of moles of the solute and the mass of the solvent in kg. When we divide the number of moles of the solute by the mass of the solvent in kg, we get the molality of the solution. The formula is given below:
$Molality=\frac{Moles\text{ }of\text{ }the\text{ }solute}{Mass\text{ }of\text{ }the\text{ }solvent\text{ }in\text{ }kg}$
For calculating the moles of the solute, we have to divide the given mass of the solute by the molecular mass of the solute. The formula is given below:
$Moles=\frac{Given\text{ }mass}{Molecular\text{ }mass}$
The density of pure water at room temperature i.e., ${{25}^{\circ }}C$ is 0.9970749 g/mol, therefore the mass will be 0.9970749 kg, and the molecular mass is 18.0148 g/mol.
$Moles=\frac{997.0749\text{ g}}{18.0148\text{ g/mol}}=55.348$
So, the molality will be:
$Molality=\frac{55.348}{0.9970749}=55.510\text{ m}$
The molality is 55.510 m.

Note: Since there is no volume in the calculation of the molality of the solution, so the molality is temperature independent and does not change even on changing the temperature of the solution.