A gas is collected by bubbling it into an inverted, water-filled tube standing in a trough of water. (a) Name the piece of apparatus labelled X used to meas...

This question tests two ways of collecting a gas, why a soluble gas gives a low reading, and how a rate is read off a volume-time graph.

(a) The graduated tube used to measure the collected volume of gas over water is a measuring cylinder (accept gas measuring tube). [1]

(b) An advantage of a gas syringe over collecting over water is that a gas syringe does not let the gas dissolve, gives a more accurate reading, and has no water in the way (any one). [1]

(c) Collecting carbon dioxide over water can give a volume that is too small because carbon dioxide is soluble in water, so some of it dissolves and is not collected. [1]

(d) The reaction of the carbonate with the acid:

calcium carbonate + hydrochloric acid \(\rightarrow\) calcium chloride + water + carbon dioxide

One mark for the reactants, one for all three products. [2]

(e) To find how the rate changes with time, record the volume of gas at regular time intervals and then plot a graph of volume of gas against time. Two marks. [2]

(f) On that graph, the rate at any moment is shown by the gradient (steepness) of the curve: the steeper the curve the faster the rate. Two marks. [2]

Exam takeaway: collect a soluble gas such as carbon dioxide or sulfur dioxide with a gas syringe, not over water, or the measured volume will always read low.