Question

A pH 4 buffer solution is prepared by dissolving one mole of a weak acid HA (pKa = 4.0) and one mole NaA in 1 L of water. When the volume is increased to 10 L by adding distilled water, the pH of the resulting solution is closest to:_____________

Answer 1

pH is closest to 4.0

Step-by-step explanation:

According to Henderson-Hasselbalch equation for a buffer consist of an weak acid (HA) and it's conjugate base (
`A^(-)`)-

`pH=pK_(a)(HA)+log([A^(-)])/([HA])`

where
`[A^(-)]` and [HA] represents concentration (in molarity) of
`A^(-)` and HA respectively.

We know,
`molarity=(n_(solute))/(V_(solution)in liter)`

Number of moles of HA and
`A^(-)` remains same even after diluting the solution to 10 L.

So,
`([A^(-)])/([HA])=((n_(A^(-)))/(V_(solution)in liter))/((n_(HA))/(V_(solution)in liter))=((1 mol)/(10 L))/((1mol)/(10L))=1`

So,
`pH=4.0+log(1)=4.0`

Hence pH is closest to 4.0