What is the pH of a solution with hydrogen ion concentration of 1 × 10⁻⁴ mol/dm³?

## The reasoning pH measures how acidic or basic a solution is, and it's calculated using this formula: **pH = −log₁₀[H⁺]** Where [H⁺] is the hydrogen ion concentration. Given: [H⁺] = 1 × 10⁻⁴ mol/dm³ Substituting into the formula: pH = −log₁₀(1 × 10⁻⁴) pH = −(−4) *(because log₁₀ of 10⁻⁴ = −4)* **pH = 4** The negative sign in the formula flips that −4 to positive 4. ## Why the wrong options tempt you **Option A (2):** You might mistakenly square the exponent or confuse this with a different concentration like 10⁻². **Option C (10) & D (14):** These come from forgetting the negative sign or mixing up with pOH calculations. Remember, pH + pOH = 14, but that's a different calculation. Also, 10 and 14 would indicate basic solutions, but with lots of H⁺ ions, this solution is clearly acidic. ## Quick takeaway **Count the zeros after the decimal in standard form — that's your pH!** For 10⁻⁴, the exponent 4 *is* the pH. Simple!