The Student Room Group

this question is from edexcel unit 5 chemistry january 2014 ial

9 10 cm3
of a gaseous hydrocarbon was mixed with excess oxygen and ignited. The
gas volumes were measured at room temperature and pressure before and after
combustion and it was found that the total gas volume had contracted by 20 cm3
.
Given that combustion was complete, the formula of the hydrocarbon was
A C4
H4
B C4
H6
C C4
H8
D C4
H10
i dont know how to approach this question
Original post by riamu
9 10 cm3
of a gaseous hydrocarbon was mixed with excess oxygen and ignited. The
gas volumes were measured at room temperature and pressure before and after
combustion and it was found that the total gas volume had contracted by 20 cm3
.
Given that combustion was complete, the formula of the hydrocarbon was
A C4
H4
B C4
H6
C C4
H8
D C4
H10
i dont know how to approach this question

If you expect help the least you could do is to format the question so that it is readable. Did you look at what you had posted?
Reply 2
Original post by charco
If you expect help the least you could do is to format the question so that it is readable. Did you look at what you had posted?

well for that very reason i have given details above about the paper and the the year
Original post by riamu
well for that very reason i have given details above about the paper and the the year

Very helpful ...

... if someone just happens to have that paper.
Reply 4
Original post by riamu
10 cm3 of a gaseous hydrocarbon was mixed with excess oxygen and ignited. The gas volumes were measured at room temperature and pressure before and after combustion and it was found that the total gas volume had contracted by 20 cm3.

Given that combustion was complete, the formula of the hydrocarbon was
A C4H4
B C4H6
C C4H8
D C4H10

I don't know how to approach this question

It took me less than ten seconds to fix.
Original post by riamu
9 10 cm3
of a gaseous hydrocarbon was mixed with excess oxygen and ignited. The
gas volumes were measured at room temperature and pressure before and after
combustion and it was found that the total gas volume had contracted by 20 cm3
.
Given that combustion was complete, the formula of the hydrocarbon was
A C4
H4
B C4
H6
C C4
H8
D C4
H10
i dont know how to approach this question

Notice that all of the possible answers have four carbon atoms.
The difference between the reactant gases and the final volume is given
The reactant gases are made up of 10cm3 hydrocarbon + total oxygen
The product gases are made up of carbon dioxide and excess oxygen
Notice that there are four carbon atoms in each of the possible answers, so there is 40 cm3 of carbon dioxide produced.

Now can you see how to proceed?
Reply 7
Original post by charco
Notice that all of the possible answers have four carbon atoms.
The difference between the reactant gases and the final volume is given
The reactant gases are made up of 10cm3 hydrocarbon + total oxygen
The product gases are made up of carbon dioxide and excess oxygen
Notice that there are four carbon atoms in each of the possible answers, so there is 40 cm3 of carbon dioxide produced.

Now can you see how to proceed?

Okay help me break the question down

It’s says total gas volume contracts by 20cm3 so does that mean 10cm3 of hydrocarbon and the other 10cm3 from oxygen,right
Original post by riamu
Okay help me break the question down

It’s says total gas volume contracts by 20cm3 so does that mean 10cm3 of hydrocarbon and the other 10cm3 from oxygen,right

Do you understand the word "contract"?
Reply 9
Original post by charco
Do you understand the word "contract"?

Yes, I do
Reply 10
https://i.imgur.com/u9fljP3.jpeg

could some help me with this with a detailed calculation of how to find out the formula of the compound
What does the shaking with potassium hydroxide do?
Reply 12
Original post by charco
What does the shaking with potassium hydroxide do?

Absorb carbon dioxide
Reply 13
Original post by riamu
Absorb carbon dioxide

https://i.imgur.com/UgVpa5w.jpg


if you could please go through my calculation and pin where i am going wrong
(edited 3 years ago)
Original post by riamu
https://i.imgur.com/UgVpa5w.jpg


if you could please go through my calculation and pin where i am going wrong

If 10cm3 of hydrocarbon makes 40 cm3 of carbon dioxide
then 1 mol of hydrocarbon makes 4 mol of carbon dioxide and there are four carbon atoms in the formula.

1. The total initial volume = volume of hydrocarbon(=10cm3) + total oxygen
2. The total final volume = excess oxygen + carbon dioxide (=40cm3)

and

3. total final volume - total initial volume = 20cm3 (because it contracts by 20cm3)

1. V(initial) = 10 + Oxygen(total)
2. V(final) = 40 + Oxygen(excess)
3. V(initial) - V(final) = 20cm3
4. Oxygen(reacted) = Oxygen(total) - Oxygen(excess)

Hence:
Oxygen(reacted) = V(initial) - 10cm3 - V(final) + 40cm3
Oxygen(reacted) = V(initial) - V(final) - 10cm3 + 40cm3
Oxygen(reacted) = V(initial) - V(final) + 30cm3
from 3.
V(initial) - V(final) = 20cm3
Oxygen reacted = 20cm3 + 30cm3 = 50cm3

We already know that 4 mol of carbon react with 4 mol of oxygen
Hence all of the hydrogen reacts with 1 mol of oxygen

2H2 + O2 ==> 2H2O

Hence there are four hydrogen atoms in the molecule.

C4H4
Original post by charco
If 10cm3 of hydrocarbon makes 40 cm3 of carbon dioxide
then 1 mol of hydrocarbon makes 4 mol of carbon dioxide and there are four carbon atoms in the formula.

1. The total initial volume = volume of hydrocarbon(=10cm3) + total oxygen
2. The total final volume = excess oxygen + carbon dioxide (=40cm3)

and

3. total final volume - total initial volume = 20cm3 (because it contracts by 20cm3)

1. V(initial) = 10 + Oxygen(total)
2. V(final) = 40 + Oxygen(excess)
3. V(initial) - V(final) = 20cm3
4. Oxygen(reacted) = Oxygen(total) - Oxygen(excess)

Hence:
Oxygen(reacted) = V(initial) - 10cm3 - V(final) + 40cm3
Oxygen(reacted) = V(initial) - V(final) - 10cm3 + 40cm3
Oxygen(reacted) = V(initial) - V(final) + 30cm3
from 3.
V(initial) - V(final) = 20cm3
Oxygen reacted = 20cm3 + 30cm3 = 50cm3

We already know that 4 mol of carbon react with 4 mol of oxygen
Hence all of the hydrogen reacts with 1 mol of oxygen

2H2 + O2 ==> 2H2O

Hence there are four hydrogen atoms in the molecule.

C4H4


Im sure theres an easier way of doing this hence the 1 mark?
Original post by JohnJ123
Im sure theres an easier way of doing this hence the 1 mark?

The key thing to note is that all molecules have four carbons. 10cm3 of hydrocarbon forms 40cm3 of CO2. Total volume has gone down by 20cm3 which represents the water that has formed. Assuming all volumes are moles, then 4 moles of CO2 forms 2 moles of H2O. We have four carbons, we must have four hydrogens (2 x H2O)

Quick Reply

Latest