# Chemistry Paper 6b Edexcel

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16 years ago
#1
2AI) A Fluoride of Phosphorous Y, contains 24.6% by mass of phosphrous and has a molar mass of 126gmol-1 Deduce the Molecular formula.

2c) when hydrogen fluoride is dissolved in water a solution of a weak acid is formed. write the expression for the acid disocciation constant, Ka, for hydrogen fluroide. Calculate the value of Ka, with units given that 0.150 mol dm-3 solution of hydrogen flouride has a pH of 2.04.

1b) what indicator did you use and why?

1c)Suggest a Chemical test to confirm the presence of ammonia in solution X.
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16 years ago
#2
Hi there, I just did Edexcel Chem 6B as well. Here are the answers I got for the questions you posted:

2a) i) Ar(P) = 31 Ar(F) = 19
24.6% P thus 100 - 24.6 = 75.4% F
24.6/31 = 0.794 75.4/19 = 3.968
0.794/0.794 = 1 3.968/0.794 = 5
Thus molecular formula = PF5
This is confirmed by the molar mass of 126: 31 + 5(19).

2c) Ka = [H+][F-]/[HF]
Ka = [H+]^2/[HF]
[H+] = 10^-2.04 = 0.00912 mol dm^-3
Ka = 0.00912^2/0.150 = 5.55 x 10^-4 mol dm^3

1b) Methyl orange. Turns from colourless to orange (orange below pH 5), fits vertical pH range between 5 and 3.5.

1c) Use red litmus. Turns from red to blue with the presence of ammonia.
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16 years ago
#3
What did you think of the exam?
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16 years ago
#4
1b) Methyl orange. Turns from colourless to orange (orange below pH 5), fits vertical pH range between 5 and 3.5.

1c) Use red litmus. Turns from red to blue with the presence of ammonia.

That aint accurate
Methyl Orange goes from RED to YELLOW

Use DAMP RED LITMUS On top of where NH3 Gas is released and gas turns BLUE indicating ALKALINE gas for NH3.
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16 years ago
#5
Ok, fair enough about the colour changes. The dampness for the litmus paper doesn't have to be explicit though.
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16 years ago
#6
this was my test: soak X in cotton wool, and another with HJCL, bring them together, white fumes should form.

Is that OK?
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16 years ago
#7
easiest synoptic i have seen for a while, even tho im **** at chem Question 2 was a gimme, and so was the majority of 4 although i think i screwed up the Kc part of it by doing pressing the wrong buttons on my calulator even tho im pretty sure i had the right working out - d'oh! Whats the grade boundaries for unit 6 - ive heard about 77/100 for an A?
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16 years ago
#8
Originally posted by Unregistered
easiest synoptic i have seen for a while, even tho im **** at chem Question 2 was a gimme, and so was the majority of 4 although i think i screwed up the Kc part of it by doing pressing the wrong buttons on my calulator even tho im pretty sure i had the right working out - d'oh! Whats the grade boundaries for unit 6 - ive heard about 77/100 for an A?
Yeah, considering they have such high grade boundaries for this paper, it's really hard to get A.
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