A water having pH value equal to 9 will have hydrogen ion concentration equal to
- A10^–9 moles / litreCorrect
- B109 moles / litre
- C10^–5 moles / litre
- D105 moles / litre
Solution
Concept
pH is the negative log (base 10) of hydrogen ion concentration.
Formula
pH = -log10[H^+] so [H^+] = 10-pH
Step-by-step solution
1. Given pH = 9. 2. [H^+] = 10-pH. 3. [H^+] = 10-9 moles/litre.
Explanation
pH tells us the power of 10 of the hydrogen ion concentration with a negative sign. For pH 9, the concentration is 10-9 moles per litre. This is a basic (alkaline) water.
Why the other options are wrong
B and D have positive powers, which is not possible for dilute water here. C (10-5) is for pH 5.
Shortcut
[H^+] = 10-pH directly.
Common mistake
Forgetting the negative sign in the power.
Correct answer: A) 10^–9 moles / litre