A buffer solution contains a weak acid, HA , and its conjugate base, A − . The buffer solution has a pH of 5.76 , and the weak acid has a p K a of 5.31 . Without performing a calculation, determine the relationship between the concentration of the weak acid and the concentration of the conjugate base in this buffer solution? The relationship between [ HA ] and [ A − ] cannot be determined.

The relationship is expressed as follows:

Explanation:

Most acidic substances are weak acids and are therefore only partially ionized in acqeous solution. We cab use the equilibrium constant for the ionization of acid to express the extent to which the weak acid ionizes. If we represent a general weak acid as HA, we can write the equation for its ionization reaction like this:

To calculate the pH of a weak acid, we use the equilibrium concentration of the reacted species and product.

Take for example:

HA → H + A⁻

where A id the conjugate base.

Knowing that x amount of acid reacts, we can solve like this:

HA → H + A⁻

H+ = antilog (pH)

thus, the pH of the acid is equals to H+ (initial) - H+ (equilibrium) ≈ H+ (initial)