The pH of a solution changes from 2 to 5. What happens to the hydrogen ion concentration?
Answer and explanation
Correct answer: It decreases one thousand times
The governing relation is pH = −log₁₀[H⁺]. Therefore, an increase of one pH unit means a tenfold decrease in hydrogen-ion concentration. The change from pH 2 to pH 5 is 5 − 2 = 3 units, so [H⁺] decreases by 10³ = 1000 times. Equivalently, the concentration changes from 10⁻² to 10⁻⁵ mol L⁻¹. Hence option C is correct; A and B count too few units, while D reverses the direction.
Frequently asked questions
What is the correct answer to this question?
It decreases one thousand times
Why is this the correct answer?
The governing relation is pH = −log₁₀[H⁺]. Therefore, an increase of one pH unit means a tenfold decrease in hydrogen-ion concentration. The change from pH 2 to pH 5 is 5 − 2 = 3 units, so [H⁺] decreases by 10³ = 1000 times. Equivalently, the concentration changes from 10⁻² to 10⁻⁵ mol L⁻¹. Hence option C is correct; A and B count too few units, while D reverses the direction.
Which subject and chapter does this question cover?
This is a Class 10 Science question. Chapter: Chemical Substances – Nature and Behaviour. Topic: Acids, Bases, and Salts.
