pH, Kw and strong alkalis

Science · Chemistry · ages 17-18

Name ______________________ Date ____________

You dilute an acid tenfold. What happens to its pH?
- It falls by one unit
- It stays exactly the same
- It rises by one unit
What is the pH of 0.01 mol/dm3 hydrochloric acid?
- 1
- 2
- 12
Pure water at 50 C has a pH below 7, so it must be acidic.
Circle one: True False
What is [H+] in 0.10 mol/dm3 NaOH at 25 C, with Kw = 1.0 x 10-14?
- 1.0 x 10-13 mol/dm3
- 1.0 x 10-7 mol/dm3
- 0.10 mol/dm3
A strong acid has [H+] = 0.001 mol/dm3. What is its pH?
Answer: ______________
A solution has pH 3. What is its hydrogen ion concentration?
- 3 mol/dm3
- 0.001 mol/dm3
- 30 mol/dm3
A solution has pH 2. What is its [H+] in mol/dm3?
Answer: ______________
A student finds the pH of 0.05 mol/dm3 NaOH by taking -log(0.05). What is the mistake?
- That log gives pOH-style maths on the wrong ion, since [H+] must come from Kw first
- The concentration should be multiplied by ten before the log
- NaOH has no effect on pH at all

Answer key

For grown-ups. Fold this page away before handing over the rest.

It rises by one unit · A tenth of the [H+] means one pH unit higher on the log scale.
2 · pH = -log(0.01) = 2. The acid is fully split, so [H+] equals 0.01.
False · Kw grew with warming, but [H+] still equals [OH-], so the water stays neutral.
1.0 x 10-13 mol/dm3 · [H+] = Kw / [OH-] = 1.0 x 10-14 / 0.10 = 1.0 x 10-13 mol/dm3.
3 · pH = -log(0.001) = 3.
0.001 mol/dm3 · pH 3 means [H+] = 10-3 = 0.001 mol/dm3.
0.01 · [H+] = 10-2 = 0.01 mol/dm3.
That log gives pOH-style maths on the wrong ion, since [H+] must come from Kw first · 0.05 is [OH-], not [H+]. Divide Kw by it first, then take the log.