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OCR Chemistry F322~ 4th June 2013~ AS Chemistry

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Reply 940

Jacksgap

Original post by music144k

hey guys,
what/how are we supposed to revise with the 'pre-release material' given?!

What is pre-released material??... thank you :smile:

Reply 941

zangorou

Original post by OldSchoolTech

I try and think of them as equations.
Once you know the M+ value, set this equal to the general formula then just evaluate to find n.

If the compound was an alcohol with M+ of 60:
General formula for alcohol CnH2n+1OH

60 = CnH2n+1OH
43 = CnH2n+1 (Subtracted 17=OH)
42 = CnH2n

CH2 = 14
42/14 = 3 = n

The put this pack into the general formula to get
C3H7OH. (propanol)

Then use other m/z peaks to determine if its propan-1-ol or 2-ol.

Hope that made sense. Seems complicated but it works for me - just do loads of practice!!!

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you are genius! :biggrin:

Reply 942

Tania2k9

let's test eachother guys!

Reply 943

Anonymous1717

In questions that ask you to calculate enthalpy change of formation from combustion values, why does oxygen have a value of 0J?

Reply 944

Gotzz

Original post by Anonymous1717

In questions that ask you to calculate enthalpy change of formation from combustion values, why does oxygen have a value of 0J?

Because oxygen is an element already so the enthalpy change for the formation of an element from an element is zero.

Reply 945

needtosucceed=)

Original post by Anonymous1717

In questions that ask you to calculate enthalpy change of formation from combustion values, why does oxygen have a value of 0J?

I think its because it doesnt take any energy to form oxygen. oxygen exists in its standard state of a gas, so to convert it into a gas takes no energy.

Reply 946

music144k

Original post by Jacksgap

What is pre-released material??... thank you :smile:

Oh, My teacher gave it to us and said we should revise from it..?
(this is for OCR salters chemistry.. sorry im new here, not sure if im on right thread..?! :confused:

)

Reply 947

Jacksgap

Original post by needtosucceed=)

I think its because it doesnt take any energy to form oxygen. oxygen exists in its standard state of a gas, so to convert it into a gas takes no energy.

Yess spot on...

well done

Reply 948

needtosucceed=)

Original post by music144k

Oh, My teacher gave it to us and said we should revise from it..?
(this is for OCR salters chemistry.. sorry im new here, not sure if im on right thread..?! :confused:

)

ah wrong thread, this is for the OCR A exam :smile:

Reply 949

music144k

Original post by needtosucceed=)

ah wrong thread, this is for the OCR A exam :smile:

Oops!

thanks! aha sorry if I confused any of you :smile:

Reply 950

zangorou

I need to learn.. remember all the pesky facts :frown:

Reply 951

Hello...

Original post by Anonymous1717

In questions that ask you to calculate enthalpy change of formation from combustion values, why does oxygen have a value of 0J?

You cannot combust oxygen so it will not have a combustion value

Reply 952

.raiden.

Original post by Gotzz

Because oxygen is an element already so the enthalpy change for the formation of an element from an element is zero.

So even though it is the combustion value they give we treat it like a formation value? :confused:

Reply 953

scorpio22

anyone revising for f324 and f325??

I have concentrated so much time on this one, i am finally happy with ppq marks but i am slacking behind f324 and f325

Reply 954

Sara_A

Original post by .raiden.

Enthalpy change = bond enthalpy of reactants - bond enthalpy of products

So

-184 = 679 - bond enthalpy of product

Bond enthalpy of product = 679 + 184
= 863

BUT this is the bond enthalpy of 2HCL
They ask for HCL so divide your answer by two. 431.5... 432 (3sf)

Hope that helped :smile:

Thanks so much, i understand it now

Reply 955

.raiden.

Original post by Hello...

You cannot combust oxygen so it will not have a combustion value

This is what I was thinking too!

Reply 956

344302

Original post by Hello...

200 Tonnes of Methane = 200,000,000g of Methane
Moles=200,000,000/16
=12500000 moles of Methane
3 Moles of Hydrogen in the equation, therefore
12500000 X 3=37500000 moles of hydrogen [THEORETICAL]

In the question it says 68.4 tonnes of Hydorgen is produced
68.4 tonnes = 68400000g of Hydrogen
Moles= 68400000/2
=34200000 moles of Hydrogen [ACTUAL]

% Yield= (34200000/37500000) X 100
=91.2%

Sorry but why did u divide the hydrogen by two?

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Reply 957

OldSchoolTech

Original post by zangorou

you are genius! :biggrin:

Aha thanks, I'm glad it made sense :P

Reply 958

Hello...

Original post by 344302

Sorry but why did u divide the hydrogen by two?

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Number of Moles= Mass/RMM
RMM of hydrogen =2

(Also i had already divided it by 1 and my overall answer was almost 200% so i knew I'd gone wrong somewhere)