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A level chemistry question help.

I’m doing a past paper question for A level chemistry and I’m stuck on a question.

Organic compound E is formed by the substitution of some of the H atoms in butane by Cl atoms.

A chemist found that 0.636g of compound E has a volume of 78.0cm3. Under the conditions used the molar has volume is 32.5dm3 mol-1.

Determine the molecular formula of compound E

The answer is C4H4Cl6 but I don’t get how you can convert the relative formula mass into a molecular formula?
(edited 2 months ago)
Reply 1
Given that it's formed from butane, you know that there are 4 carbons. Can you think of two simultaneous equations relating H and Cl?
Reply 2
I’m going to be honest but no haha I am in my first year of a level chemistry and find radical substitution a bit hard to grasp at the moment so I apologise.
Original post by Randommmm101
I’m doing a past paper question for A level chemistry and I’m stuck on a question.
Organic compound E is formed by the substitution of some of the H atoms in butane by Cl atoms.
A chemist found that 0.636g of compound E has a volume of 78.0cm3. Under the conditions used the molar has volume is 32.5dm3 mol-1.
Determine the molecular formula of compound E
The answer is C4H4Cl16 but I don’t get how you can convert the relative formula mass into a molecular formula?

Your organic compound formula is impossible.

1.

Work out the moles using the molar volume given.

2.

Work out the molar mass using the mass and the moles.

3.

Using the carbon skeleton for butane, determine the number of chlorine atoms that there must be.

Reply 4
Original post by Randommmm101
I’m going to be honest but no haha I am in my first year of a level chemistry and find radical substitution a bit hard to grasp at the moment so I apologise.

No worries :smile:

If we let x be the number of H, and y be the number of Cl...

... then we can make one equation based on how many atoms we know there to be around a 4-carbon chain

... and a second equation involving the Mr, as we know that the contribution of the hydrogen atoms will be 1x and for chlorine it will be 35.5y
Reply 5
Original post by charco
Your organic compound formula is impossible.

1.

Work out the moles using the molar volume given.

2.

Work out the molar mass using the mass and the moles.

3.

Using the carbon skeleton for butane, determine the number of chlorine atoms that there must be.


Aye should be C4H4Cl6
Reply 6
Original post by charco
Your organic compound formula is impossible.

1.

Work out the moles using the molar volume given.

2.

Work out the molar mass using the mass and the moles.

3.

Using the carbon skeleton for butane, determine the number of chlorine atoms that there must be.

I made a typo and put a 1 so it was actually meant to be C4H6Cl6
Reply 7
Original post by Methene
Aye should be C4H4Cl6
Haha oops 😂
Reply 8
Original post by Methene
No worries :smile:
If we let x be the number of H, and y be the number of Cl...
... then we can make one equation based on how many atoms we know there to be around a 4-carbon chain
... and a second equation involving the Mr, as we know that the contribution of the hydrogen atoms will be 1x and for chlorine it will be 35.5y
Ohh that makes so much more sense now and muchmore simpler than what I was thinking of haha butthank you so much for your help! Your explanation was comprehensible!
Reply 9
Original post by charco
Your organic compound formula is impossible.

1.

Work out the moles using the molar volume given.

2.

Work out the molar mass using the mass and the moles.

3.

Using the carbon skeleton for butane, determine the number of chlorine atoms that there must be.

Thank you!! I got the first and second bit but the 3rd part tripped me up.
Reply 10
Original post by Randommmm101
Ohh that makes so much more sense now and muchmore simpler than what I was thinking of haha butthank you so much for your help! Your explanation was comprehensible!

You're welcome! All the best :smile:

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