The method and the results are on the acids and bases pH lab. Do the analysis yourself first, then check the answers below.
Quick answer: A strong acid ionises completely, so its pH comes straight from its concentration. These answers go with the AP Chemistry lab on acids and bases pH: the graph, the calculations, the uncertainties and the conclusion explained, for all 7 analysis questions.
Here are the acids and bases pH lab answers for AP Chemistry: graph, calculations, uncertainties and conclusion. First do the lab yourself, then compare, and go back to the lesson if needed.
The sample data give a Ka close to the accepted value. Your own results will differ a little with the probe, temperature and dilution accuracy. Show every substitution and keep two or three significant figures in the final value.
Acids and bases pH lab answers: the analysis explained
Question 1 Hydronium concentrations
[H₃O⁺] = 10⁻ᵖᴴ. For pH 2.88: 1.32 × 10⁻³ mol/dm³. The other rows give 3.89 × 10⁻⁴ and 1.23 × 10⁻⁴ mol/dm³.
Question 2 Percent ionised
Percent = [H₃O⁺]/c × 100: 1.3 %, 3.9 % and 12.3 %. The percentage rises as the acid is diluted.
Question 3 Ka from each reading
Ka = 1.76 × 10⁻⁵, 1.57 × 10⁻⁵ and 1.73 × 10⁻⁵.
Question 4 Mean and comparison
Mean = 1.69 × 10⁻⁵. This differs from 1.8 × 10⁻⁵ by 6 %, so the results agree well.
Question 5 Predicting the next dilution
Solve x² + 1.8 × 10⁻⁵x − 1.8 × 10⁻⁹ = 0: x = 3.437 × 10⁻⁵ mol/dm³, so pH = 4.46. The shortcut would be inaccurate here because x is about 34 % of c.
Question 6 Pipetting error
The real concentration is 0.0099 mol/dm³, 1 % lower than assumed. Using c = 0.0100 in the formula makes c − [H₃O⁺] slightly too big, so the calculated Ka is about 1 % too low.
Question 7 Random error
Probe not at equilibrium: wait until the reading is steady for 30 seconds. Carbon dioxide dissolving in the dilute solutions: keep the beaker covered and measure quickly. Repeat each reading three times and average.
