Question

Define molarity and molality and explain its difference?

Answer 1

Hint: Molarity is the ratio of moles of solutes in per litre of solution while the molality is the ratio of moles of solutes in per kilogram of solvent.
Complete answer:

Molarity, which is denoted by 'M' is defined as the number of moles of solute in per litre solution and its unit is (moles/litre) which is one of the most common units which is used to measure the concentration of solution.
Molarity is generally used to calculate the volume of solvent or the amount of solute.
The formula for molarity is:
${\text{M = }}\dfrac{{\text{n}}}{v}$
where ${\text{M}}$ is molarity or molar concentration,${\text{n}}$ is moles of solute and ${\text{v}}$ is litres of solution.
The relationship between two solutions having same amount of moles of solute is shown by the formula ${{\text{c}}_1}{{\text{v}}_1}{\text{ = }}{{\text{c}}_2}{{\text{v}}_2}$ where ${\text{c}}$ is concentration i.e. molarity and ${\text{v}}$ is volume.
Now comes to the term Molality, which is denoted by 'm' is defined as the number of moles of solute in per kilogram of solvent and its SI unit is (mol/kg).
\[{\text{m = }}\dfrac{{{\text{moles of solute}}}}{{{\text{kg of solvent}}}}\]
The basic difference between the two is that molarity is the measurement of volume while molality is the measurement of mass.
Molarity depends on the volume of solution and molality is an intensive property of solutions which depends only on the mass of solvent and also molarity varies according to temperature and pressure as volume of solution changes if there is a variation in these parameters while on the other side using the weight is an advantage because mass does not shows variation with these parameters so molality can be used in a range of temperatures.

Note: Both are the methods which represent the concentration of solution, molarity represents the molar concentration in terms of volume of solution while molality represents the molal concentration in terms of mass of solvent.