Empircal formula ?

I was asked to work out the following:

Compound X contains only boron and hydrogen. The percentage by mass of boron X is 81.2%. In the mass spectrum of X the peak at the largest value m/z occurs at 54.

Calculate the empircal formula.

I worked it out like this:

81.2/10.8=7.5 18.8/1=18.8 However the answer says: B2 :h:

5 ?!?!
7.5/7.5=1 18.8/7.5=2.5

1: 2

It then says deduce the molecular mass - answer says its: B4: H10!?

Reply 1

EierVonSatan

21

18.8/7.5 = 2.5 so the ratio is 1: 2.5 which can't exist so it must be 2:5, if it was 2.1 you could round down but its too far out for errors

Reply 2

Woody.

12

The start of what you've done is right

81.2/10.8 = 7.5 18.8/1 = 18.8

That's a 7.5:18.8 ratio

Divide by the smallest number

7.5/7.5 = 1
18.8/7.5 = 2.5

That's a 1:2.5 ratio

The empirical formula is the simplest whole number ratio, 2.5 is not a whole number

Get it to a whole number by multiplying by 2

That's a 2:5 ratio

Therefore, B2H5

Reply 3

Malsy

18

Sure it didn't ask deduce the molecular formula?

Reply 4

Baby Princess

OP

Ohhh, so if it was 2.6 would the ratio be 2:6?

Reply 5

Baby Princess

OP

Malsi101

Sure it didn't ask deduce the molecular formula?

Yes lol, appologies!

Reply 6

Malsy

18

Baby Princess

Yes lol, appologies!

Reply 7

Malsy

18

Baby Princess

Ohhh, so if it was 2.6 would the ratio be 2:6?

no lol..they wouldn't normally give you 2.6 the reason why 2.5 is 5 is because you have to double it because it's not a whole number.

so

1:2.5
would be
2:5

because you've doubled 1 to get 2 and 2.5 to get 5.

Reply 8

Baby Princess

OP

Ahhh okay thanks lol, it makes sense!! It confused me ALOT and was thinking godamnit my revisions just going to waste !!!!

Reply 9

Woody.

12

The most complicated I've seen is like, 2:3.33 and you have to times it by three. I don't think they'll give you something like 2.7, you'd just have to assume that that's wrong.

Reply 10

AVAINTEX

6

You missed a step
First use an accurate calculator i recommend you to buy the Casio fx-991 Ex or a higher version

1:you subtracts 100-(81.2)= 18.2% Hydrogen

2:number of moles of B= 81.2 g / 11=7.38 mol B
number of moles of H= 18.2 g / 1 =18.2 mol H

3 : Divide by the Fewest number of moles.<That would be the Boron Moles
B=7.38/7.38=1
H=18.2/7.38=2.4

4: Now here is a trick some student dont know H=2.4 >> the fraction is 0.4 to solve this or any other Empirical formula problem follow these steps

a. 1/4(the fraction) =0.25

5: Gongrat you solved your ordeal :P

take the 0.25 and divide the

c.B = 1 / 0.25 = 4

d.H = 2.4 / 0.25 = 9.6 which equal 10

SO MF is B4H10

SOLVED

Reply 11

AVAINTEX

6

Lol its not that complicated ive seen worse. I study the Chang College book believe me that book is a nightmare but i carry it a lot it is My reference.

Reply 12

AVAINTEX

6

Woody.

The start of what you've done is right

81.2/10.8 = 7.5 18.8/1 = 18.8

That's a 7.5:18.8 ratio

Divide by the smallest number

7.5/7.5 = 1
18.8/7.5 = 2.5

That's a 1:2.5 ratio

The empirical formula is the simplest whole number ratio, 2.5 is not a whole number

Get it to a whole number by multiplying by 2

That's a 2:5 ratio

Therefore, B2H5

Excuse me But it is not the right way.Ive Already explained how you can solve it.

Extra Empirical formula problem:
Ascorbic acid(vitamin C) Cures scurvy.it is composed of 40.92% carbon,4.58% Hydrogen,54.50% Oxygen.Determine its Empirical Formula.

GOOD LUCK :biggrin:

Reply 13

Woody.

12

AVAINTEX

Excuse me But it is not the right way.Ive Already explained how you can solve it.

Extra Empirical formula problem:
Ascorbic acid(vitamin C) Cures scurvy.it is composed of 40.92% carbon,4.58% Hydrogen,54.50% Oxygen.Determine its Empirical Formula.

GOOD LUCK :biggrin:

Uhh, firstly I explained how to do it before you. It clearly is the right way as myself, EierVonSatan, and the mark scheme have the same answer. It's the way I've been taught by qualified Chemistry teachers and I am inclined to believe that way rather than yours as I don't really know you nor your method.