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pH curve for strong acid with weak base Checking Answers

The question is the attachment and I was wondering if people got the same answers and in a few cases actually know the answer:

10

a)

Equation occuring between point A and B: CO32(aq)- + H+(aq) ===> HCO3-(aq)

Equation occuring between point C and D: HCO3- (aq) + H+ (aq) ==>H2CO3 (aq)

b) This is at point E so the minimum volume of HCl need is about 30 cm3.

c) Indicator appropriate for points between D and E for strong acid/weak base (very approximately about 7 - 3 pH); so appropriate indicator is methyl orange.

I am not entirely sure why this indicator would not change colour from points B to C. Could some explain this please?

Is it to do with a reduction in [H+] so the equilibrium shifts to the left (the reverse reaction) favouring HIn?

d) 2HCl (aq) + Na2CO3 (aq) ===> 2NaCl (aq) + CO2(g) + H20(l)

So the number of moles of HCl = (0.12 x 30.0) / 1000 = 3.6 x 10^-3 mol

Moles of Na2CO3 = Moles of HCl / 2

So moles of Na2CO3....3.6 x 10 ^-3 / 2 = 1.8 x 10^-3 mol

Concentration of Na2CO3 = (1.8 x 10^-3 x 1000) / 25 = 0.072 mol dm-3

e) I have no idea by lecturer has not covered this type of calculation so please could someone explain this to me!

NB: This is a repost sorry but nobody seem to see the question.

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Reply 1

b_shaikha4321

methly orange works well around ph 5-7 at end point, so it would not work at point B to C as the ph value are too high. the rest are fine but e im not sure either, as i do the edexcel board and our sylabus are different.