Answers section

Questions 2.7(b)

Sulphuric (VI) acid is normally poured carefully into water when preparing its aqueous solution and not the other way round. Explain this. (Hint: Sulphuric acid produces large quantities of heat (highly exothermic) as it dissolves in water. Water has a lower boiling point than the acid.)
Explain why the limb of a volumetric flask is made narrow.
What is the advantage of using a wash bottle when topping up the level of a solution?
Concentrated sulphuric acid has a density of 1.836 g/ml. What volume of the acid should be used to prepare 250 ml of 0.2M solution?
What is the volume of water in 250 ml of 0.2M sulphuric acid?
Suggest a reason why, for liquid solutes, we usually measure volume rather than mass when preparing their solutions.

Answers to Questions 2.7(b)

The heat generated with each small volume of acid added to water is quickly absorbed by the large volume of water so temperature does not rise so high as to cause boiling. But when water is added (in small volumes), the same amount of heat is absorbed by smaller amount of water whose temperature quickly rises to boiling point, causing violent explosion that splashes the acid around.
A small limb ensures that a small amount of distilled water added causes a large increase in level; so it is sensitive and gives an accurate measurement.
Wash bottle releases water in small amounts for better control of the level, ensuring the mark is not overshot.
1000 ml would require 0.2 moles. Therefore 250 ml requires (250/1000) x 0.2 = 0.05 moles. Because RMM of H₂SO₄ is 1x2 + 32 + 16x4 = 98, 0.05 moles weigh 0.05 x 98 = 4.9 g
But density = 1.836 g/ml; so 1.836g occupies 1 ml. Therefore, 4.9g occupies (4.9/1) x 1.836 = 8.996, practically 9.00 ml.
250-9.00 = 241 ml
The liquids are corrosive and any spill readily attacks metal and parts of weighing balance. It is more convenient and easier to measure volume and, unlike powdered solids, liquids have no empty spaces in between that would cause errors in measurement.