This question is about a titration experiment carried out by a group of students to determine the concentration of a solution of ethanoic acid using sodium hydroxide.
A student weighed about 4.00 g of sodium hydroxide pellets and added them to a beaker containing 50 cm3 of deionised water.
The mixture was stirred with a glass rod to dissolve the pellets and to give a homogenous solution.
The solution was poured through a funnel into a 250.0 cm3 volumetric flask and deionised water was added up to the mark and then the flask was shaken.
(2)
State why the procedure has to be restarted rather than using a teat pipette to remove the excess water.
(1)
Two students each cleaned a burette, then poured sodium hydroxide solution into their burettes.
Give two steps needed before the student takes the initial burette reading.
(2)
Give the effect, if any, on the value of the first titre. Justify your answer.
(1)
The sketch shows the pH changes during a titration of 25.0 cm3 of ethanoic acid with sodium hydroxide of the same concentration.
The ideal indicator for this titration will change colour on the addition of a very small volume of sodium hydroxide solution at a titre value very close to the equivalence point of the reaction.
(4)
(1)
Indicator | pH range | Tick or Cross |
Bromocresol purple | 5.2 – 6.8 | |
Thymol blue | 8.0 – 9.6 | |
Thymolphthalein | 8.3 – 10.6 | |
Alizarin yellow R | 10.1 – 13.0 |
Each student used a pipette to measure 25.0 cm3 of the ethanoic acid solution into four separate conical flasks and added an indicator.
The results of one student’s titrations are shown in the table.
Titration number | 1 | 2 | 3 | 4 |
Final burette reading / cm3 | 13.00 | 25.50 | 37.90 | 50.00 |
Initial burette reading / cm3 | 0.25 | 13.00 | 25.50 | 37.90 |
Titre / cm3 | ||||
Concordant titres (✓) |
(1)
Calculate the total percentage measurement uncertainty if the burette had been refilled to 0.00, and then a further 0.30 cm3 had been added from the burette, to the conical flask.
The measurement uncertainty for each burette reading is ±0.05 cm3.
(1)
The teacher carried out the experiment and obtained the following results.
Mass of sodium hydroxide used to make 250.0 cm3 solution = 3.80 g
Volume of ethanoic acid solution = 25.00 cm3
Mean titre of sodium hydroxide = 11.90 cm3
CH3COOH (aq) + NaOH (aq) → CH3COONa (aq) + H2O (l)
Calculate the concentration of the ethanoic acid solution in g dm−3.
Give your answer to an appropriate number of significant figures.
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