Hard AS calculation questions .. :/ Watch

Adorable98
Badges: 15
#1
Report Thread starter 3 years ago
#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 =
0
reply
Adorable98
Badges: 15
#2
Report Thread starter 3 years ago
#2
Bump!!
0
reply
RKM21
Badges: 3
Rep:
?
#3
Report 3 years ago
#3
what spec is this for?
0
reply
Adorable98
Badges: 15
#4
Report Thread starter 3 years ago
#4
I'm doing Edexcel but found this question on the internet..
0
reply
Adorable98
Badges: 15
#5
Report Thread starter 3 years ago
#5
BUMP!!
0
reply
nwmyname
Badges: 13
Rep:
?
#6
Report 3 years ago
#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
1
reply
Adorable98
Badges: 15
#7
Report Thread starter 3 years ago
#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!!!
0
reply
X

Quick Reply

Attached files
Write a reply...
Reply
new posts
Latest
My Feed

See more of what you like on
The Student Room

You can personalise what you see on TSR. Tell us a little about yourself to get started.

Personalise

University open days

  • Norwich University of the Arts
    Postgraduate (MA) Open Day Postgraduate
    Thu, 18 Jul '19
  • Bournemouth University
    Clearing Open Day Undergraduate
    Wed, 31 Jul '19
  • Staffordshire University
    Postgraduate open event - Stoke-on-Trent campus Postgraduate
    Wed, 7 Aug '19

Are you tempted to change your firm university choice now or on A-level results day?

Yes, I'll try and go to a uni higher up the league tables (131)
19.35%
Yes, there is a uni that I prefer and I'll fit in better (67)
9.9%
No I am happy with my course choice (383)
56.57%
I'm using Clearing when I have my exam results (96)
14.18%

Watched Threads

View All