Question 3
- Outline how a 250 cm3 of 0.100 moldm-3 HCl could be prepared from 100 cm3 of a 2.00 moldm-3 stock solution.
- State the effect of aqueous solutions of each of the following salts on litmus paper:
(i) CH3COONa;
(ii) Al2(SO4)3;
(iii) KNO3. - (c) In a redox titration experiment, acidified potassium tetraoxomanganate (VII) [KMnO4] solution was titrated against Fe2+ solution. Which solution was placed in the burette?
Observation
The question was popular among the candidates.
In part (a), majority of the candidates did not apply dilution law. So they did not get the answer.
In part (b), majority of the candidates stated inferences instead of observations.
In part (c), majority of the candidates correctly wrote the solution that was placed inside the burette.
The expected answers include:
-
Method:
Pipette / measure 12.50 cm3 of the stock solution into a 250 cm3 volumetric flask (containing some distilled water). Shake / swirl and add more distilled water until the mark is reached. - (i) CH3COONa - turns red litmus blue
(ii) Al2(SO4)3 - turns blue litmus red
(iii) KNO3 - no effect on litmus / neutral to litmus - (Potassium tetraoxomanganate (VII) / KMnO4 solution