Quick questions on Buffer systems explained: HSC Chemistry Module 5

For an acetic acid / acetate buffer:

A buffer has a finite capacity. Once enough acid is added to consume all the conjugate base (or enough base to consume all the weak acid), the buffer fails and the pH changes rapidly. Capacity is maximised when $[HA]$ and $[A^-]$ are equal and both large.

Blood is buffered between pH 7.35 and 7.45 by the carbonic acid / bicarbonate system:

Identify the two components of an effective buffer and explain why a strong acid and its conjugate base would not buffer effectively. [3 marks]

A buffer is prepared by mixing 0.20 mol $CH_3COOH$ ($pK_a = 4.74$) with 0.10 mol $CH_3COONa$ in 1.0 L of water. Calculate the buffer pH. [3 marks]

A buffer at pH 7.4 is prepared from $H_2PO_4^-$ / $HPO_4^{2-}$ ($pK_a = 7.21$). (a) Calculate the ratio $[HPO_4^{2-}] / [H_2PO_4^-]$. (b) Write equations to show how the buffer responds to added $H_3O^+$ and added $OH^-$.