Introduction

When acids and bases mix, determining the final pH involves understanding which reactant is in excess. This guide explains how to calculate pH in acid-base reactions, a crucial concept in chemistry.

Key Principles

The pH of a solution after mixing an acid and base depends on which reactant remains in excess:

Step-by-Step Calculation Method

Worked Example

Let's solve this problem: Calculate the pH when mixing 30 mL of 0.10 M NaOH with 50 mL of 0.30 M HCl.

Step 1: Write the balanced equation

$\text{HCl}_{(aq)} + \text{NaOH}_{(aq)} \rightarrow \text{NaCl}_{(aq)} + \text{H}_2\text{O}_{(l)}$

Step 2: Calculate moles of reactants

For NaOH: $n(\text{NaOH}) = c \times V = 0.10 \text{ M} \times 0.030 \text{ L} = 0.0030 \text{ mol}$

For HCl: $n(\text{HCl}) = c \times V = 0.30 \text{ M} \times 0.050 \text{ L} = 0.015 \text{ mol}$

Step 3: Determine excess reactant

Step 4: Calculate final concentration

Total volume = 80 mL = 0.080 L

Final HCl concentration: $[\text{HCl}] = \frac{0.012 \text{ mol}}{0.080 \text{ L}} = 0.15 \text{ M}$

Step 5: Calculate pH

Since HCl is a strong acid: $[\text{H}^+] = [\text{HCl}] = 0.15 \text{ M}$ $\text{pH} = -\log(0.15) = 0.82$

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