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

Answer:

pH is closest to 4.0

Explanation:

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

$pH=pK_{a}(HA)+log\frac{[A^{-}]}{[HA]}$

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

We know, $molarity=\frac{n_{solute}}{V_{solution}in liter}$

Number of moles of HA and $A^{-}$ remains same even after diluting the solution to 10 L.

So, $\frac{[A^{-}]}{[HA]}=\frac{\frac{n_{A^{-}}}{V_{solution}in liter}}{\frac{n_{HA}}{V_{solution}in liter}}=\frac{\frac{1 mol}{10 L}}{\frac{1mol}{10L}}=1$

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

Hence pH is closest to 4.0