You are Here: Home

# Hard AS calculation questions .. :/

Announcements Posted on
TSR's new app is coming! Sign up here to try it first >> 17-10-2016
1. 1) 10cm3 of a hydrocarbon, CxHy were exploded with an excess of oxygen. There was a contraction in volume of 30cm3. When the products were treated with sodium hydroxide (which reacts with carbon dioxide), there was a further contraction of 30cm3. Deduce the formula of the hydrocarbon, given that all the volumes were measured under the same conditions.
So What I did so far:
CxHy +O2 -> CO2 + H2O
CxHy = 10 cm3
O2 =
CO2 =
H2O =
2. Bump!!
3. what spec is this for?
4. I'm doing Edexcel but found this question on the internet..
5. BUMP!!
6. Well, from the information given, you are told that 10cm3 of hydrocarbon was burnt.

10CxHy + some O2 → some CO2 + some H2O

You are told that the volume contracts by 30cm3
Note water turns into a liquid when burnt as it condenses when it climatises.

This means that 10cm3 + Some O2 - Some CO2 = 30cm3

You know you have 10cm3 of the hydrocarbon so:
Some O2 - Some CO2 = 20cm3

Hold on here! Volume further contracted by 30cm3 when Sodium Hydroxide reacted

This means that
Some O2 - 20cm3 = 30cm3 → Some O2 = 60cm3

You know have an equation.

10CxHy + 50 O2 → 30 CO2 + some H2O
Divide by 10 gives CxHy + 5O2 → 3CO2 + some H2O
You have 12 oxygens on one side and you know that 6 oxygens are used on the right hand side for 3CO2 so 4 oxygens are left for the H2O.

This gives 4H2O.
CxHy + 5O2 → 3CO2 + 4H2O
Having 3 carbons on the left side gives x = 3. You have 8 hydrogen atoms on the right hand side, so y = 8.

Final Formula C3H8
7. (Original post by nwmyname)
Well, from the information given, you are told that 10cm3 of hydrocarbon was burnt.

10CxHy + some O2 → some CO2 + some H2O

You are told that the volume contracts by 30cm3
Note water turns into a liquid when burnt as it condenses when it climatises.

This means that 10cm3 + Some O2 - Some CO2 = 30cm3

You know you have 10cm3 of the hydrocarbon so:
Some O2 - Some CO2 = 20cm3

Hold on here! Volume further contracted by 30cm3 when Sodium Hydroxide reacted

This means that
Some O2 - 20cm3 = 30cm3 → Some O2 = 60cm3

You know have an equation.

10CxHy + 50 O2 → 30 CO2 + some H2O
Divide by 10 gives CxHy + 5O2 → 3CO2 + some H2O
You have 12 oxygens on one side and you know that 6 oxygens are used on the right hand side for 3CO2 so 4 oxygens are left for the H2O.

This gives 4H2O.
CxHy + 5O2 → 3CO2 + 4H2O
Having 3 carbons on the left side gives x = 3. You have 8 hydrogen atoms on the right hand side, so y = 8.

Final Formula C3H8
Oh I see!! Thank you soo much!!!

## Register

Thanks for posting! You just need to create an account in order to submit the post
1. this can't be left blank
2. this can't be left blank
3. this can't be left blank

6 characters or longer with both numbers and letters is safer

4. this can't be left empty
1. Oops, you need to agree to our Ts&Cs to register

Updated: June 9, 2016
TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

This forum is supported by:
Today on TSR

### How does exam reform affect you?

From GCSE to A level, it's all changing

### Who would you like to thank?

Poll
Study resources

The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE

Reputation gems: You get these gems as you gain rep from other members for making good contributions and giving helpful advice.