AQA AS Jun 2016 Paper 2 Q8 (with explained solutions) |

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AQA AS Jun 2016 Paper 2 Q8

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18When an aqueous solution of ethanoic acid reacts with magnesium, the progress of reaction can be followed using the equipment shown in Figure 5 to measure the volume of hydrogen produced. Figure 5 Figure 6 shows how the volume of hydrogen produced varies with time when 396 mg of magnesium are added to 30.0 cm3 of 0.600 mol dm3 ethanoic acid. Time M/JUN16/7404/2 Figure 6 Volume of hydrogen *18*<br />
190 8 . 1 The equation for the reaction between ethanoic acid and magnesium is shown. 2CH3COOH(aq) + Mg(s)(CH3COO)2Mg(aq) + H2(g) With the aid of calculations, show that the magnesium is in excess in this reaction. [3 marks] 0 8 . 2 The reaction was repeated using 20 cm3 of 0.800 mol dm3 of ethanoic acid solution with all other conditions the same. The magnesium was still in excess. Sketch a line on Figure 6 to show how the volume of hydrogen produced varies with time in this second experiment. [2 marks] Space for working. Turn over for the next question *19* Turn overM/JUN16/7404/2<br />

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MARK SCHEMEAS-LEVEL CHEMISTRY 7404/2JUNE 2016 Marking Guidance Mark Comments Question 08.1 Method 1 M1 Moles of Mg = 0.396/24.3 = 0.0163 M2 Moles of CH3COOH = 0.600 x 30.0/1000 = 0.018 M3 Mark for showing Mg is in excess: either 0.018 mol of CH3COOH reacts with 0.009 mol of Mg OR 0.0163 mol of Mg reacts with 0.0326 mol of CH3COOH OR 0.0073 mol of Mg is in excess Method 2 M1 Moles of CH3COOH = 0.600 x 30.0/1000 = 0.018 M2 Moles of Mg that would react with this = 0.009 M3 Mass of Mg needed = 24.3 x 0.009 = 0.219 g which is less than 0.396 g OR Moles of Mg = 0.0163 which is more than 0.009 required Method 3 M1 Moles of Mg = 0.396/24.3 = 0.0163 M2 Moles of CH3COOH that would react with this = 0.0326 M3 Volume of CH3COOH needed = 0.0326 / 0.60 = 0.0543 dm3 (54.3 cm3) which is more than 0.030 dm3 (30 cm3) 1 1 Allow working throughout to 2sf If candidate gets 16.3 mol (as not converted mg to g) in method 1 or 3 then can only score 1 mark maximum (M2) Accept other valid calculations that show the Mg is in excess 08.2 M1 Line starts at origin and is steeper M2 (moles CH3COOH = 0.800 x 20/1000 = 0.016) line levels out on 8th line up (line below the original 9th line) 1 1 M2 for line on 8th line on grid (original on 9th line)allow some leniency so long as clear it ends at (or very close to) the 8th line; and line does not significantly wobble 23 of 24<br />

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