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Author Topic: Edexcel CHEMISTRY DOUBTS!!!!  (Read 253774 times)

Offline Meticulous

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Re: Edexcel CHEMISTRY DOUBTS!!!!
« Reply #30 on: May 10, 2010, 09:32:22 pm »
People sometimes don't see things. No need to go around showing off your smartness.
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Offline ksitna

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Re: Edexcel CHEMISTRY DOUBTS!!!!
« Reply #31 on: May 10, 2010, 10:00:10 pm »
why do i keep getting 0.6 then???
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Offline ksitna

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Re: Edexcel CHEMISTRY DOUBTS!!!!
« Reply #32 on: May 10, 2010, 10:00:57 pm »
no actually 0.0006
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Offline Saladin

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Re: Edexcel CHEMISTRY DOUBTS!!!!
« Reply #33 on: May 11, 2010, 03:40:02 am »
Quote from: The Gladiator on May 10, 2010, 04:58:35 pm
I need help with this question:

" 25 cm3 of potassium iodate (V) solution was added to excess potassium iodide solution. The iodine liberated required 30 cm3 of 0.5 mol dm-3 sodium thiosulphate. Calculate the concentration of the potassium iodate (V) solution."

Needed ASAP.

Thanks in advance.

Can you please give the exact question paper for this, I need to see the equations of the reaction to give you the answer.
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Offline Summer :]

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Re: Edexcel CHEMISTRY DOUBTS!!!!
« Reply #34 on: May 11, 2010, 06:10:36 am »
Quote from: The Mysterious Dude on May 10, 2010, 09:09:12 am
You guys are wrong.

You see, the Bromine and Iodine ions are very powerful reducing agennts compared to that of Chlorine, and would result in further reactions, that would not form other products. http://www.chemguide.co.uk/inorganic/group7/halideions.html#top. This site will help u understand the whole thing.

with Bromine, it would just form HBR, MHSO4 (m for the metal), Br2, SO2, H20
check the sulphur's oxidation number in MHSO4 its (+6) and in S02 (its +4) - it has been reduced furthermore where there was a decrease in the oxidation number
with Iodine because its a stronger reducing agent it would cause the sulphur to be reduced further more
so H2S is formed..
again check the sulphur's oxidation number MHSO4 (+6) and in S02 (+4) and H2S (-2) - so has been reduced alot due the to the stronger reducing agent which is Iodine.
but you dont need that for the organic part, just the part about redox reactions of halogens..
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Offline Blizz_rb93

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Re: Edexcel CHEMISTRY DOUBTS!!!!
« Reply #35 on: May 11, 2010, 10:21:50 pm »
This question is from my mock exam but i noticed some questions when i began revising i didnt know how to do it..
so please if anyone can help me understand these questions

2) This question is about two isomeric alcohols, X and Y, each with molar mass 60g mol-1

A solution of potassium dichromate(VI) in dilute sulfuric acid is added to each alcohol. Both alcohols cause the same color change of the mixture on heating.
a) A colorless liquid, B, is distilled from the mixture containing alcohol X.

The liquid B forms a red precipitate when it is boiled with Benedict's or Fehling's solution.
Give the displayed formula of liquid B, and the name of alcohol X.

Liquid B




Name of alcohol X _______________________________

b) A colorless liquid, C, is distilled from the mixture containing alcohol Y.
C does not react when it is boiled with Benedict's or Fehling's solution
Give the names of liquid C and alcohol Y

Liquid C __________________________
Alcohol Y ___________________________

__________________________________________________________________________

That's for question 2, here's question 3.. i know it's kinda long :( but i need help on this so im very sorry

3) This question is about Calcium hydroxide, Ca(OH)2

The solubility of calcium hydroxide in water can be found by titrating a saturated solution of calcium hydroxide with hydrochloric acid of known concentration.
a) Describe how you would make a saturated solution of calcium hydroxide suitable for use in this titration. Do not describe the subsequent titration procedure.

b) 10.0 cm3 portions of the saturated solution are placed in a conical flask and titrated with 0.0500 mol dm-3 hydrochloric acid added from a burette
i) Name the apparatus used to measure the 10.0 cm3 portions

ii) Suggest a suitable indicator for this titration and state the color change you would expect to see at the end point.

c) The following results were obtained

Titration numbers |                                 1 |                                    2 |                                      3    |
_____________________________________________________________________________

Final burette reading/cm3 | 19.20             |         28.05                     |          37.10                         |
_____________________________________________________________________________
Initial burette reading/cm3 | 10.00            |       19.20                      |         28.15                          |
_____________________________________________________________________________
Titre/ cm 3                          |  9.20             |     8.85                           |            8.95                             |
____________________________________________________________________________

i) suggest why the first titre should be disregarded.

The rest i know

Thank you all in advance! <33
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Offline vanibharutham

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Re: Edexcel CHEMISTRY DOUBTS!!!!
« Reply #36 on: May 12, 2010, 04:44:08 am »
This question is about two isomeric alcohols, X and Y, each with molar mass 60g mol-1
This means that they are isomers of C3H8O

A solution of potassium dichromate(VI) in dilute sulfuric acid is added to each alcohol. Both alcohols cause the same color change of the mixture on heating.
They both were oxidized so it cant be a tertiary alcohol

a) A colorless liquid, B, is distilled from the mixture containing alcohol X.

The liquid B forms a red precipitate when it is boiled with Benedict's or Fehling's solution.
This means that it has been oxidized further so liquid B must be propanal
CH3CH2CHO
And alcohol X must be CH3CH2CH2OH


b) A colorless liquid, C, is distilled from the mixture containing alcohol Y.
C does not react when it is boiled with Benedict's or Fehling's solution
Not oxidized further so it must be Propanone - CH3C=OCH3
And the alcohol Y must be a secondary alcohol CH3CH(OH)CH3

« Last Edit: May 12, 2010, 05:04:43 am by vanibharutham »
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Offline vanibharutham

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Re: Edexcel CHEMISTRY DOUBTS!!!!
« Reply #37 on: May 12, 2010, 05:04:11 am »
3) This question is about Calcium hydroxide, Ca(OH)2

The solubility of calcium hydroxide in water can be found by titrating a saturated solution of calcium hydroxide with hydrochloric acid of known concentration.
a) Describe how you would make a saturated solution of calcium hydroxide suitable for use in this titration. Do not describe the subsequent titration procedure.
Not sure about this one....
Add a known amount of Ca(OH)2 to water and keep stirring. When no more dissolves start heating it. When the solution begins to boil keep adding more Ca(OH)2 while you are still heating it, until no more dissolves.


b) 10.0 cm3 portions of the saturated solution are placed in a conical flask and titrated with 0.0500 mol dm-3 hydrochloric acid added from a burette
i) Name the apparatus used to measure the 10.0 cm3 portions
Pipette

ii) Suggest a suitable indicator for this titration and state the color change you would expect to see at the end point
Phenolphthalein... It is in basic conditions at the start so it will be purple. At the end point it will be colourless

i) suggest why the first titre should be disregarded.
the titre should be disregarded as it was probably a check titration. Furthermore, the titre was more that 0.1cm³ off from the other two values. Titration is a precision method of volumetric analysis and it should be disregarded because it is too far off from the other values
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Offline Meticulous

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Re: Edexcel CHEMISTRY DOUBTS!!!!
« Reply #38 on: May 12, 2010, 10:57:18 am »
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Offline Tammy

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Re: Edexcel CHEMISTRY DOUBTS!!!!
« Reply #39 on: May 12, 2010, 12:02:58 pm »
i have a question about percentage error.....

The balance used to weigh the magnesium carbonate is accurate to +-0.01 g. Calculate the percentage error in the mass of the magnesium carbonate weighed. ((weighed MgCO3: 0.21g))

the correct answer is 0.02
                             -----
                               0.21

Why is it 0.02 not 0.01?? and when do we write it as 0.01??   

Thanks in advance peeps
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Offline Blizz_rb93

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Re: Edexcel CHEMISTRY DOUBTS!!!!
« Reply #40 on: May 12, 2010, 12:52:54 pm »
How do we calculate the solubility of a substance???
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Offline Saladin

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Re: Edexcel CHEMISTRY DOUBTS!!!!
« Reply #41 on: May 12, 2010, 01:39:10 pm »
Quote from: Psyclax on May 12, 2010, 12:52:54 pm
How do we calculate the solubility of a substance???

It is the number of grams that can be dissolved divided by the volume of water. You do not need to calculate it for the examination.
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Offline ksitna

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Re: Edexcel CHEMISTRY DOUBTS!!!!
« Reply #42 on: May 12, 2010, 02:23:07 pm »
Quote from: vanibharutham on May 12, 2010, 04:44:08 am
This question is about two isomeric alcohols, X and Y, each with molar mass 60g mol-1
This means that they are isomers of C3H8O

A solution of potassium dichromate(VI) in dilute sulfuric acid is added to each alcohol. Both alcohols cause the same color change of the mixture on heating.
They both were oxidized so it cant be a tertiary alcohol

a) A colorless liquid, B, is distilled from the mixture containing alcohol X.

The liquid B forms a red precipitate when it is boiled with Benedict's or Fehling's solution.
This means that it has been oxidized further so liquid B must be propanal
CH3CH2CHO
And alcohol X must be CH3CH2CH2OH


b) A colorless liquid, C, is distilled from the mixture containing alcohol Y.
C does not react when it is boiled with Benedict's or Fehling's solution
Not oxidized further so it must be Propanone - CH3C=OCH3
And the alcohol Y must be a secondary alcohol CH3CH(OH)CH3


liquid b is propan-1-ol
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Offline vanibharutham

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Re: Edexcel CHEMISTRY DOUBTS!!!!
« Reply #43 on: May 12, 2010, 02:30:28 pm »
Quote from: ksitna on May 12, 2010, 02:23:07 pm
liquid b is propan-1-ol

Nope, liquid B is obtained from Distillation of Alcohol X

X = Propan-1-ol
B = Propanal
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Offline ksitna

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Re: Edexcel CHEMISTRY DOUBTS!!!!
« Reply #44 on: May 12, 2010, 02:42:29 pm »
markscheme says propanol :/
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