AS & A Level Chemistry - 9701 Paper 3 2016 Summer Zone 2

In Questions 1 and 2 you will determine the relative atomic mass, $A_r$, of magnesium by two different methods. In the first method you will collect and measure the volume of gas given off in the reaction between a known mass of magnesium and a known amount of dilute sulfuric acid. The acid will be in excess.

$$ \text{Mg(s) + H}_2\text{SO}_4\text{(aq)} \rightarrow \text{MgSO}_4\text{(aq)} + \text{H}_2\text{(g)} $$

FB 1 is a strip of magnesium ribbon, Mg. FB 2 is 1.00 mol $ ext{dm}^{-3}$ sulfuric acid, $ ext{H}_2 ext{SO}_4$.

(a) Method

Read through the whole method before starting any practical work. The diagrams below may help you in setting up your apparatus.



• Fill the tub with water to a depth of about 5 cm.
• Fill the 250 $ ext{cm}^3$ 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 the open end is in the water just above the base of the tub.
• Pipette 25.0 $ ext{cm}^3$ of FB 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 $ ext{cm}^3$ measuring cylinder.
• Weigh the magnesium ribbon, FB 1, and record the mass in the space below.
• Remove the bung from the neck of the flask. Add the magnesium ribbon, FB 1, into the acid in the flask and replace the bung immediately. Remove the flask from the clamp and swirl it to mix the contents. Swirl the flask occasionally until no more gas is evolved. Replace the flask in the clamp.
• When no more gas is collected, measure and record the final volume of gas in the measuring cylinder in the space below.

KEEP THE CONTENTS OF THE CONICAL FLASK X FOR USE IN QUESTION 2.

Results

(b) Calculations

Show your working and appropriate significant figures in the final answer to each step of your calculations.

(i) Calculate the number of moles of gas collected in the measuring cylinder. [Assume that 1 mole of gas occupies 24.0 $ ext{dm}^3$ under these conditions.]

moles of gas = ......................... mol

(ii) Use your answer to (i) and the mass of magnesium, FB 1, recorded in (a) to calculate the relative atomic mass, $A_r$, of magnesium.

$A_r$ of magnesium = .........................

You will determine the amount of sulfuric acid remaining in flask X after the reaction with magnesium in Question 1. You will do this by titration with sodium hydroxide of known concentration.

2NaOH(aq) + H_2SO_4(aq) → Na_2SO_4(aq) + 2H_2O(l)

FB 3 is 0.150 mol dm^{-3} sodium hydroxide, NaOH.
bromophenol blue indicator

(a) Method

• Transfer all the contents of flask X from Question 1 into the 250 cm^3 volumetric flask.
• Rinse flask X with distilled water and add the washings to the volumetric flask. Add distilled water up to the mark. Stopper the volumetric flask and mix the contents thoroughly.
• Label this solution FB 4.
• Rinse the pipette and use it to transfer 25.0 cm^3 of FB 4 into the conical flask.
• Add about 10 drops of bromophenol blue to the conical flask.
• Fill the burette with FB 3.
• Perform a rough titration and record your burette readings in the space below. The end point is reached when the solution becomes a permanent blue-violet colour.

• Carry out as many accurate titrations as you think necessary to obtain consistent results.
• Record, in a suitable form below, all of your burette readings and the volume of FB 3 added in each accurate titration.
• Make certain any recorded results show the precision of your practical work.

(a) (i) FB 5 is a solid element and FB 6 is a solid compound containing one cation and one anion. Carry out the following tests and record your observations.

[Table_1]

(ii) Suggest the identities of FB 5 and FB 6 from your observations. Refer to the relevant observations in your answers.

FB 5: .................................................................
reason(s) .................................................................
..........................................................................

FB 6: cation .......................................... reason(s) ...........................................................
..........................................................................
anion .......................................... reason(s) ........................................................
..........................................................................

(b) (i) FB 7 and FB 8 are solutions which contain different anions. These may be carbonate, halide, sulfate or sulfite. You are to devise tests to identify the two anions present. Record in a suitable table below:
• the reagent(s) you use for each test,
• the observations you make on carrying out the test,
• the conclusion you make from the result of the test.

(ii) The cation in either FB 7 or FB 8 is a transition metal ion. Carry out the following test to identify this cation and record your observations.

[Table_2]

FB ......... contains the transition metal ion ....................... .