Question

Calculate the formality of NaCl solution, 5.85 g of which have been dissolved to form 250 ml of the given solution.

Answer 1

Hint: Formality of a solution can be defined as the number of gram formula masses of solute is dissolved in per litre of the solution. Formality of a solution is represented by the symbol F. Formality is somewhat similar to normality.

Complete Step by step solution: Generally the term formality is used to express the concentration of the ionic solids which do not exist as molecules but exist as networks of ions. A solution containing one gram formula mass of solute per litre of the solution has formality equal to one and is called formal solution.
Formula mass of any substance can be defined as the sum of the atomic masses of all the atoms in a formula unit of a compound. It can also be known by the name formula weight. Formula used to calculate formality is:
$Formality=\dfrac{Number\ \text{of}\ \text{gram}\ \text{formula}\ \text{masses}\ \text{of}\ \text{solute}}{Volume\ \text{of}\ \text{solution}\ \text{in litre}}$
It can also be calculated with the number of moles in the given compound i.e.
$Formality=\dfrac{M\text{oles}\ \text{of}\ \text{solute}}{Volume\ \text{of}\ \text{solution}\ \text{in litre}}$
$Moles\ \text{of}\ \text{NaCl}=\dfrac{Given\ \text{weight}}{Molecular\ \text{weight}}$
$Moles\ \text{of}\ \text{NaCl}=\dfrac{5.85}{58.5}=0.1$
Volume of solution = 250 ml (Given) = 0.25 L
$Formality=\dfrac{0.1}{0.25}=0.4F$

Hence the formality of NaCl solution, 5.85 g of which have been dissolved to form 250 ml of the given solution is 0.4 F.

Note: Formality of any solution is temperature dependent quantity i.e. formality of a solution changes with change in temperature. The purpose of using the term formality is to distinguish the number of moles of the compound to the number of moles of ions in solution of ionic compound or weak electrolyte.