In this experiment you will determine the concentration of a sample of hydrochloric acid. You will do this by measuring the volume of hydrogen produced when an excess of magnesium reacts with the acid. Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g) FA 1 is magnesium powder, Mg. FA 2 is hydrochloric acid, HCl.

1. (a)Method • Weigh the container with FA 1. Record the mass. • Fill the tub with water to a depth of approximately 5 cm. • Fill the 250 cm³ measuring cylinder completely with water. Hold a piece of paper towel firmly over the top, invert the measuring cylinder and place it in the water in the tub. • Remove the paper towel and clamp the inverted measuring cylinder so that the open end is just above the base of the tub. • Use the 25 cm³ measuring cylinder to place 25.0 cm³ of FA 2 into the reaction flask, labelled X. • Check that the bung fits tightly in the neck of flask X, clamp flask X, and place the end of the delivery tube into the inverted 250 cm³ measuring cylinder. • Remove the bung from the neck of flask X. Tip all of FA 1 into flask X and replace the bung immediately. Remove the flask from the clamp and swirl to mix the contents. • Swirl the flask occasionally until no more gas is evolved. Replace the flask in the clamp. • Measure and record the final volume of gas in the measuring cylinder. • Weigh and record the mass of the container with any residual solid. • Calculate and record the mass of FA 1 used. Keep FA 2 for use in Question 2.[2]
2. (b)(i)Calculate the number of moles of hydrogen gas produced. (Assume 1 mol of gas occupies 24.0 dm³ at this temperature.) moles of H2(g) = .............................................. mol [1][1]
3. (b)(ii)Calculate the concentration of hydrochloric acid in FA 2. concentration of HCl in FA 2 = .............................................. mol dm⁻³ [1][1]
4. (b)(iii)In this experiment the magnesium powder was in excess. Calculate the mass of magnesium powder needed for complete reaction with all the hydrochloric acid in 25.0 cm³ of FA 2. mass of Mg = .............................................. g [1][1]
5. (c)A student suggested two modifications to the method in (a) to give a more accurate value for the concentration. For each suggestion, state whether you agree with the student and explain your answer. Suggestion 1: Use magnesium ribbon rather than powdered magnesium; keep the rest of the experiment the same. Suggestion 2: Use twice the mass of magnesium powder; keep the rest of the experiment the same. [2][2]
6. (d)Another student carried out the experiment in (a) but used less magnesium than that calculated in (b)(iii). State and explain the effect this would have on the calculated concentration of hydrochloric acid in FA 2. [1][1]