Question

What is formality? Explain with example.

Answer 1

Formality:

• “It is defined as the number of gram formula mass of solute dissolved in per litre of solution.” It is used for ionic compounds.
• The formality of the solution is denoted by the symbol, $\mathrm{F}$.
• It is similar to normality.

Formula used to calculate the formality:

• $\text{Formality}=\frac{\text{Number of gram formula mass of solute}}{\text{Volume of solution in litre}}$

Or,

• $\text{Formality}=\frac{\text{Moles of solute}}{\text{Volume of solution in litre}}$

Or,

• $\text{Formality}=\frac{\text{Given weight of solute}}{\text{Molecular weight of solute}\times \text{Volume of solution in litre}}$

For example:

Calculate the formality of sodium chloride $\left(\mathrm{NaCl}\right)$ solution, $5.85\mathrm{g}$ of which have been dissolved to form $250\mathrm{mL}$ of the given solution.

Step 1: Given,

• Weight of solute $\left(\mathrm{NaCl}\right)=5.85\mathrm{g}$
• Molecular weight of solute $\left(\mathrm{NaCl}\right)=58.5\mathrm{g}{\mathrm{mol}}^{-1}$
• Volume of solution $=250\mathrm{mL}=0.250\mathrm{L}(\because 1\mathrm{L}=1000\mathrm{ml})$

Step 2: To determine the formality of $\mathrm{NaCl}$ solution.

• $\text{Formality}=\frac{\text{Given weight of solute}}{\text{Molecular weight of solute}\times \text{Volume of solution in litre}}$
• $\text{Formality}=\frac{5.85\mathrm{g}}{58.5\mathrm{g}{\mathrm{mol}}^{-1}\times 0.250\mathrm{L}}$
• $\text{Formality}=0.4\mathrm{F}$

Therefore, the formality of $\mathrm{NaCl}$ solution is $0.4\mathrm{F}$.