Answer to Question #2684 in Physical Chemistry for Noreena

Question

1.6 g ( an excess) of Mg was added to 100 cm[sup]3[/sup] of 2.0 mol dm[sup]–3[/sup] CuSO4(aq). The temperature rose from 20C to 65C (45’C rise). Find ΔHr for the reaction: (c = 4.2 J g[sup]-1[/sup] K[sup]-1[/sup])
Mg(s) + CuSO[sub]4[/sub](aq) --> MgSO[sub]4[/sub](aq) + Cu(s)

Find the energy change by using ΔH = - mcΔT
(MY ANSWER) 100 x 4.2 x 45 = 18900 J mol[sup]-1[/sup] = 18.9 kJ mol[sup]-1[/sup] (which is correct)

Find how many moles reacted (n)
(MY ANSWER) 1.6/159.6 = 0.01 then 0.01/2 = 0.005 (which is incorrect, correctly should be 2x100/1000; do not know how!)

Scale the quantities for the combustion of 1 mole (ΔH/n)
18.9/0.005 = -3780 kJ mol[sup]-1[/sup] (using the previous incorrect figure this is also wrong)

Could you please help to explain step 2?

Answer 1

Mass of magnesium given to excess. Therefore, to calculate the amount of matter we have to use the amount of copper sulfate.

n(CuSO₄) = (2 mol/dm³)*100cm³/1000 = 0.005 mol