Formality & Strength | Concentration Terms Class 11

Formality & Strength of Solution

Specific Concentration Terms | Basic Concepts of Chemistry

1. Formality ($F$)

Formality is very similar to Molarity, but it is specifically used for Ionic Compounds (like $NaCl$, $KNO_3$) which exist as formula units rather than discrete molecules.

                    Definition: The number of gram-formula masses (Formula Weights) of the ionic solute dissolved in 1 Liter of the solution.
                

$$ F = \frac{\text{Mass of Solute ($w$)}}{\text{Formula Mass ($FW$)}} \times \frac{1000}{V_{(mL)}} $$

Example: For NaCl, we use Formula Mass (58.5 g) instead of "Molecular Mass". Numerically, $F \approx M$ for strong electrolytes, but $F$ is theoretically more correct for ionic solids.

Temperature Effect: Depends on Volume ($V$), so Formality changes with temperature.

2. Strength of Solution ($S$)

This is the most direct way of expressing concentration in quantitative analysis (titrations).

Definition: The amount of solute in grams present in 1 Liter of the solution.

$$ S = \frac{\text{Mass of Solute (g)}}{\text{Volume of Solution (L)}} $$

Unit: $g/L$ or $g \cdot dm^{-3}$.

3. Important Relations

A. Strength vs Molarity ($M$)

$$ S = \text{Molarity} \times \text{Molar Mass} $$ $$ S (g/L) = M (mol/L) \times M_{wt} (g/mol) $$

B. Strength vs Normality ($N$)

$$ S = \text{Normality} \times \text{Equivalent Mass} $$ $$ S (g/L) = N (eq/L) \times E_{wt} (g/eq) $$

C. Formality vs Molarity

For molecular compounds (e.g., Glucose), $F = M$.
For ionic compounds, Formality uses observed Formula Mass, whereas Molarity often implies the theoretical molecular mass.

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