A 10 mL sample of 0.1 M HCl is mixed with 20 mL of 0.1 M NaOH. Write the balanced chemical equation and calculate the pH of the resulting solution.

medium 2 marks Class 7 · Science · Acids, Bases and Salts

Answer

pH ≈ 12.5

The correct answer is pH ≈ 12.5 — Finally, pH: $14 - 1.48 = 12.52$ (≈12.5).

Step-by-step solution

Write the balanced equation: $d i s pl a y s t y l ece H C l + N a O H - > N a C l + H 2 O$ .
Calculate moles: $ce H C l : 0.1 M \times 0.010 L = 0.001 mol$ ; $ce N a O H : 0.1 M \times 0.020 L = 0.002 mol$ .
Determine excess base: $0.002 - 0.001 = 0.001 mol$ $ce N a O H$ .
Find final volume: $10 mL + 20 mL = 30 mL = 0.030 L$ .
Compute $ce O H^{-}$ concentration: $\frac{0.001 mol}{0.030 L} = 0.0333 M$ .
Find pOH: $- l o g_{10} (0.0333) = 1.48$ .
Finally, pH: $14 - 1.48 = 12.52$ (≈12.5).

Common mistake: Neglecting volume conversion to litres or ignoring excess base in pH calculation

Concept tested: pH calculation after mixing