A student is provided with magnesium ribbon, dilute hydrochloric acid of concentration 2.0 mol/dm³, distilled water, and the apparatus shown in Fig. 11.1. T...

This is a planning question on how acid concentration affects rate, followed by measuring the hydrogen given off: \( \text{Mg} + 2\text{HCl} \rightarrow \text{MgCl}_2 + \text{H}_2 \). The key skills are diluting a stock acid fairly and controlling every other variable.

(a) Apparatus X, which measures the volume of hydrogen gas produced, is the gas syringe [1].

(b) A workable method scores up to four marks [4]: dilute the 2.0 mol/dm³ acid with measured volumes of distilled water to make a range of lower concentrations [1]; keep the total volume of acid plus water the same each time, for example always making 50 cm³, so the amount of liquid is fair [1]; use the same length or mass of magnesium ribbon each time [1]; add the acid, start the stopclock and record the volume of gas collected at a fixed time, or record the time to collect a fixed volume [1]; then repeat for each concentration. For example, mixing 25 cm³ of the 2.0 mol/dm³ acid with 25 cm³ of water gives a 1.0 mol/dm³ acid.

(c) Any two variables kept constant [2]: the length or mass of magnesium, the temperature, the total volume of liquid, and using the same apparatus.

(d) The concentration that gives the largest volume of gas in the fixed time, or that reaches a fixed volume in the shortest time, reacted fastest [1].