Empirical Formula help

Hi, I am doing this question and needed help on how a ratio would be calculated to get the empirical formula.
I keep getting an empirical formula of CH3, the mark scheme says C3H7.
Here is the question:

Analysis of a different hydrocarbon Y shows that it contains 83.7% by mass of carbon.
Calculate the empirical formula of Y.
Use this empirical formula and the relative molecular mass of Y (Mr = 86.0) to
calculate the molecular formula of Y.

Reply 1

shannon181

3

83.7% of carbon means there is 16.3% of hydrogen because there is only carbon and hydrogen in hydrocarbon

Divide by RAM so 83.7/12 = 6.975 and 16.3/1 = 16.3

Divide by smallest number so 6.975/6.975 = 1 and 16.3/6.975 = 2.34

2.34 isn't near enough to 3 to round up and I think that's where you're making the mistake

So multiply both by 3 to get 1x3 = 3 and 2.34 x 3 = 7

Giving empirical formula C3H7

Reply 2

GoldenShade

OP

13

Original post by shannon181

83.7% of carbon means there is 16.3% of hydrogen because there is only carbon and hydrogen in hydrocarbon

Divide by RAM so 83.7/12 = 6.975 and 16.3/1 = 16.3

Divide by smallest number so 6.975/6.975 = 1 and 16.3/6.975 = 2.34

2.34 isn't near enough to 3 to round up and I think that's where you're making the mistake

So multiply both by 3 to get 1x3 = 3 and 2.34 x 3 = 7

Giving empirical formula C3H7

Ahhh, that makes sense, thanks a lot!!

Reply 3

Sreekar2001

1

Original post by shannon181

83.7% of carbon means there is 16.3% of hydrogen because there is only carbon and hydrogen in hydrocarbon

Divide by RAM so 83.7/12 = 6.975 and 16.3/1 = 16.3

Divide by smallest number so 6.975/6.975 = 1 and 16.3/6.975 = 2.34

2.34 isn't near enough to 3 to round up and I think that's where you're making the mistake

So multiply both by 3 to get 1x3 = 3 and 2.34 x 3 = 7

Giving empirical formula C3H7

Thanks, ur such a babe xx

(edited 5 years ago)

Reply 4

sara.m12

16

Original post by shannon181

83.7% of carbon means there is 16.3% of hydrogen because there is only carbon and hydrogen in hydrocarbon

Divide by RAM so 83.7/12 = 6.975 and 16.3/1 = 16.3

Divide by smallest number so 6.975/6.975 = 1 and 16.3/6.975 = 2.34

2.34 isn't near enough to 3 to round up and I think that's where you're making the mistake

So multiply both by 3 to get 1x3 = 3 and 2.34 x 3 = 7

Giving empirical formula C3H7

Y would you multiply it by 3 though?

Reply 5

m23sc

1

Original post by sara.m12

Y would you multiply it by 3 though?

I think it is because you want to get to the next nearest whole number, so like if you did 2.34 x 2= 4.68 but if you do 2.34 x 3= 7.02 which is closer meaning the E.F would be C3H7.
For the molecular formula, you would divide the mr given in the Q by the mr of the worked out E.F and multipy the empirical formula by this. :smile: