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Edexcel A2 Chemistry Exams -6CH04 (14th June) and 6CH05 (22nd June) Discussion Thread

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Reply 820

ihaspotato

Mole ratio doubt -

C : H : O
1.3 : 3 : 1

should I round up 1.3 to 2 or 1?

Reply 821

C0balt

Original post by ihaspotato

Mole ratio doubt -

C : H : O
1.3 : 3 : 1

should I round up 1.3 to 2 or 1?

Multiply everything by 3?
To give r C4H9O3 I mean

Reply 822

ihaspotato

Original post by C0balt

Multiply everything by 3?
To give r C4H9O3 I mean

I took 1.3 as an example.. but there was a paper where the ratios were

1.75 : 3 :1
(x4)
giving C7H12O4

but I wanted to know if it was 1.3 - should I round it up to 1 or 2?

Reply 823

_H_V

Original post by _H_V

When calculating pH of buffer do you use the moles of the weak acid and its salt or do you use the concentration of each in the equation?
I just did a couple of buffer questions and realised I was forgetting to change the moles of the acid and salt back into concentration to put into the equation but the answer was still correct.

Anyone ?

Reply 824

C0balt

Original post by ihaspotato

I took 1.3 as an example.. but there was a paper where the ratios were

1.75 : 3 :1
(x4)
giving C7H12O4

but I wanted to know if it was 1.3 - should I round it up to 1 or 2?

Do you mean round it up to 1.0 or 2.0? I don't think you should do that because 0.3 is quite significant as opposed to 0.08 or 0.91 or something!
I think it depends how high or low the 0.3 is. I always go with gut feeling lol

Reply 825

C0balt

Original post by _H_V

Anyone ?

Concentration

Reply 826

ihaspotato

Data booklet

CH ppm 1.8-3.0 how? what does it fall under?

Reply 827

Ayman!

Original post by _H_V

Anyone ?

Perhaps the answers were still correct because the volumes cancelled?

Reply 828

Funky_Giraffe

Original post by ihaspotato

Data booklet

CH ppm 1.8-3.0 how? what does it fall under?

Can you screenshot the question? We need the context of the CH group

Reply 829

C0balt

Original post by ihaspotato

Data booklet

CH ppm 1.8-3.0 how? what does it fall under?

It's the H-c-c=o environment I believe if you look at the data booklet
This is June 15 isn't it I did it this morning

(edited 7 years ago)

Reply 830

ihaspotato

Original post by Funky_Giraffe

Can you screenshot the question? We need the context of the CH group

Reply 831

Funky_Giraffe

Original post by ihaspotato

It's the because it's the H in the H-C-C=O, which shows up at 1.8-3.0ppm

Reply 832

ihaspotato

Original post by Funky_Giraffe

It's the because it's the H in the H-C-C=O, which shows up at 1.8-3.0ppm

Ahh makes sense. Thanks! :u:

Reply 833

iwant

Original post by _H_V

Anyone ?

think of it this way, to work out conc for A- and HA u had to divide moles by volume; as the amount of volume in the solution is the same you divide both molar amounts by the same number(v), so its: A-/V divided by HA/V. As the v's cancel you are just left with the molar amounts. (A- divided by HA).

You can skip converting to concentration essentially but ka definition is using concentration values not molar amounts

Reply 834

C0balt

I think you tend to get method mark by calculating the concentration even if you don't really have to. That is often the case with Kp calculations etc, so i do every step just in case I mess up some place

Reply 835

PlayerBB

Why a solution of CH3COONa is a stronger acid than NH3 (wouldn't the ammonia dissociate to give NH4+ ?

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Reply 836

Funky_Giraffe

Original post by PlayerBB

Why a solution of CH3COONa is a stronger acid than NH3 (wouldn't the ammonia dissociate to give NH4+ ?

Yes but that also gives OH^- ions in solution. NH3 is not an acid.

Reply 837

ihaspotato

Original post by Funky_Giraffe

Totally agree with this. Although being the last main-sit exam in this series, the potential is there for them to pull out all the stops and make this unit 4 a hard one...