A 25 mL solution of nitric acid was placed in a flask and a few drops of phenolphthalein were added. Dilute sodium hydroxide was added util the indicator changed colour. It was found that 28 mL of the alkali was used.

(a) What colour was the solution in the flask at the start of the titration?
(b) What colour did it turn to when acid had been neutralized?
(c)Was the alkali more concentrated or less concentrated than the acid?

If you performed this experiment why don't you know the color before and after?

If it took more alkali (28 mL vs 25 mL HNO3), then the acid was more concentrated than the alkali.

To answer these questions, it's important to understand the principles of acid-base titrations. In this case, nitric acid (the acid) is being titrated with sodium hydroxide (the alkali) to determine the point at which the acid has been neutralized. Phenolphthalein is an indicator used to visualize this neutralization point.

(a) To determine the color of the solution in the flask at the start of the titration, we need to know the color of phenolphthalein in an acidic medium. Phenolphthalein is colorless in acidic solutions, so the solution in the flask would be colorless initially.

(b) Phenolphthalein changes color when the acid has been neutralized and an excess of hydroxide ions (OH-) is present. In this case, when the indicator changes color, it signifies that the acid has been neutralized and the solution becomes pink.

(c) To determine whether the alkali is more concentrated or less concentrated than the acid, we can use the volume measurements provided during the titration. The fact that 28 mL of the alkali (sodium hydroxide) was used to neutralize the acid indicates that the alkali was in excess. This implies that the alkali was more concentrated than the acid, as a lesser volume of alkali was required to reach the neutralization point.

In summary:
(a) The solution in the flask at the start of the titration was colorless.
(b) The solution turned pink when the acid had been neutralized.
(c) The alkali (sodium hydroxide) was more concentrated than the acid (nitric acid).