Calculate the pH of a $0.01 M$ solution of hydrochloric acid (HCl). Give your answer correct to two decimal places.

medium 3 marks Class 10 · Science · Acids, Bases and Salts

Answer

p H = 2.00

The correct answer is $p H = 2.00$ — Write the answer to two decimal places: $p H = 2.00$ .

Step-by-step solution

Recognise that HCl is a strong acid and dissociates completely: $HCl \to H^{+} + Cl^{-}$ .
Therefore the concentration of $H^{+}$ ions equals the initial concentration of HCl, i.e., $[H^{+}] = 0.01 M$ .
Apply the pH formula: $pH = - lo g_{10} [H^{+}] = - lo g_{10} (0.01)$ .
Since $lo g_{10} (0.01) = - 2$ , the pH becomes $- (- 2) = 2$ .
Write the answer to two decimal places: $p H = 2.00$ .

Common mistake: Using

p H = - lo g [H^{+}]

but forgetting that HCl dissociates completely, leading to an incorrect concentration.

Concept tested: pH of strong acid solution