500 mL of a buffer contains 0.20 M HA and 0.20 M NaA. Calculate the decrease in pH (a positive number, two decimals) on adding 0.005 mol of HCl without volume change.

Numerical answer type, tolerance 0.01.

Answer: 0.04

Moles: HA = 0.100, A

^{-}

= 0.100. After HCl: HA = 0.105, A

^{-}

= 0.095. Change in pH =

lo g (0.095/0.105) = - 0.0435

, a decrease of 0.04.