Acids and Buffers Problem Solving

Bringing It All Together

This final lesson ties together everything you have learned about acids, bases, pH, Ka, buffers, titrations, and indicators. The problems here are multi-step, combining concepts from several lessons — exactly like the extended questions you will face in your Edexcel exam.

The key to success is a systematic approach: identify the type of problem, recall the relevant equations, set out your working clearly, and check your answer makes chemical sense.

---

Multi-Step pH Calculations

Problem 1: 50.0 cm³ of 0.20 mol dm⁻³ ethanoic acid (Ka = 1.74 × 10⁻⁵ mol dm⁻³) is mixed with 25.0 cm³ of 0.20 mol dm⁻³ NaOH. Calculate the pH of the resulting solution.

Solution:

Step 1: Calculate moles.

• Moles of CH₃COOH = 0.20 × 0.0500 = 0.010 mol
• Moles of NaOH = 0.20 × 0.0250 = 0.005 mol

Step 2: The NaOH reacts with ethanoic acid: CH₃COOH + NaOH → CH₃COONa + H₂O

• NaOH is the limiting reagent (0.005 mol used up)
• Moles of CH₃COOH remaining = 0.010 - 0.005 = 0.005 mol
• Moles of CH₃COO⁻ formed = 0.005 mol

Step 3: This is a buffer (weak acid + conjugate base). Use Henderson-Hasselbalch: