**Henderson-Hasselbalch equation**.

$\overline{){\mathbf{pH}}{\mathbf{=}}{{\mathbf{pK}}}_{{\mathbf{a}}}{\mathbf{+}}{\mathbf{log}}{\mathbf{}}\mathbf{\left(}\frac{\mathbf{conjugate}\mathbf{}\mathbf{base}}{\mathbf{weak}\mathbf{}\mathbf{acid}}\mathbf{\right)}}$

• C_{2}H_{5}COONa will break up in the solution

▪ C_{2}H_{5}COO^{-} → **weak base (conjugate base)**** **** [C _{2}H_{5}COO^{-}] = [C_{2}H_{5}COONa] **

▪ Na^{+}** **→ neutral ion → will not contribute to pH

• **C _{2}H_{5}COOH** →

• The solution is composed of a weak acid and its conjugate base, therefore we have a **buffer**.

**Step 1: **Calculate the pK_{a} of the weak acid.

**Calculate pK _{a}:**

**K _{a}** C

$\overline{){{\mathbf{pK}}}_{{\mathbf{a}}}{\mathbf{=}}{\mathbf{-}}{\mathbf{log}}{\mathbf{\hspace{0.17em}}}{{\mathbf{K}}}_{{\mathbf{a}}}}\phantom{\rule{0ex}{0ex}}{\mathbf{pK}}_{\mathbf{a}}\mathbf{=}\mathbf{-}\mathbf{log}\mathbf{}\mathbf{(}\mathbf{1}\mathbf{.}\mathbf{34}\mathbf{\times}{\mathbf{10}}^{\mathbf{-}\mathbf{5}}\mathbf{)}$

**pK _{a} = 4.87**

**Step 2: **Create an ICF chart and determine the final amounts of species present after the reaction.

▪ weak acid + conjugate base = **buffer**

**Step 3: **Calculate the pH of the solution.

A buffer contains 0.16 mol of propionic acid (C_{2}H_{5}COOH) and 0.22 mol of sodium propionate (C_{2}H_{5}COONa) in 1.20 L.

What is the pH of the buffer after the addition of 0.02 mol of NaOH?

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