Question

a pure hydrocarbon gas is burned (combustion) to yield 2.641g of carbon dioxide and 1.442g of water. what is the empirical and molecular formulas.

Answer 1

first work out the amount of carbon in 2.641 g of carbon dioxide
and the amount of hydrogen in 1.442 g of water

Answer 2

Ok

Answer 3

Ok do i need to find their molar masses?

Answer 4

use the relative atomic masses
C = 12, O = 16 and H = 1.008

Answer 5

Well yeah, but I am asking f I need to find their molar masses first?

Answer 6

*if

Answer 7

Well I know that in CO2 there are 32 g of carbon dioxide, and in H2O 2.016

Answer 8

Right?

Answer 9

fraction of carbon on CO2 = 12 / (12+32)

Answer 10

Wait why would I divide the 12 by the 12+32?

Answer 11

and H in H2O is (2.016) / 18.016

Answer 12

1 C + 2 oxygens = 12 + 2*16

Answer 13

What formula would I need to use? Or what formula does this apply to

Answer 14

Oh ok, that would be the molar mass for co2

Answer 15

Right?

Answer 16

carbon dioxxide = CO2 and water = H2O

Answer 17

Oh OK thats what you call the molar mass

Answer 18

Yep

Answer 19

So I find the molar masses, and then what?

Answer 20

so multiplying those fractions by 2.641 and 1.442 will give the amounts of C and H

Answer 21

Ok, and from that how could I calculate my empirical formulas? Do I need to convert my answers to %s for % comp?

Answer 22

No you get the ratio of C to H in the hydrocarbon by dividing these 2 masses by 12 for C and 1.008 for H.

Answer 23

OH!!!

Answer 24

i get the 2 masses as 0.72027 for C and 0,16136 for H.

Answer 25

Wait, how did you get those numbers? Sorry I got a bit lost

Answer 26

divide the first by 12 and the second by 1.008 and you'l get the ratio C:H

Answer 27

C = ( 12 / 44 )* 2.641

Answer 28

oh so 12/44?

Answer 29

yea

Answer 30

work that out and it should come to 0.72027 g

Answer 31

Ok, wait. Could you pleas go over the whole thing again? I don't know why but I can;t see anything past the last post

Answer 32

Sorry, and thank you

Answer 33

Burning the hydrocarbon produces CO2 and water (H2O).

You need to find the masses of C and H in these 2 compounds.

fraction of C in CO2 = 12 (12 + 32) ( thats the molar mass of carbon / molar mass of CO2)


similar thing for H in H2O

Answer 34

Ok, could I solve really quickly and then get back to you with an answer?

Answer 35

ok

Answer 36

Ok im working on it

Answer 37

Ok wait is the amount of c in co2 .2727?

Answer 38

And would the amount of h in h2o be .119? @welshfella

Answer 39

C is 0.2727
H is 0.1119

Answer 40

yeahI got that with the 12/12+32 equations

Answer 41

Is that right?

Answer 42

yea
now you need to use these to find the amounts of C and H in the products of the reaction

Answer 43

How would I do that? Do I divide by the 2.641g and the 1.442g?

Answer 44

multiply

Answer 45

Oh.. Really I would multiply?

Answer 46

0.2727 * 2.641
0.119 * 1.442

Answer 47

* correction 0.1119 * 1.442

Answer 48

Ok

Answer 49

So for the first one the carbon I got .72

Answer 50

Would that be correct?

Answer 51

yea

Answer 52

Ok and for the hydrogen I got .17

Answer 53

0.16

Answer 54

Ok

Answer 55

Now what would I do with these two numbers?

Answer 56

now divide by the molar masses
0.72 by 12
and 0.16 by 1.008

Answer 57

Oh, ok

Answer 58

this will give you the ratio of C to H in the hydrocarbon

Answer 59

I got .06 and .15

Answer 60

Is this right?

Answer 61

0.16 is closer

Answer 62

ok

Answer 63

yeah it would be .158 so we just round it up right?

Answer 64

so ratio of C:H is 6:16

somolecular formula is C6H16

Answer 65

yes

Answer 66

oh ok

Answer 67

and then I think i now how to find the molecular formula.. I'll get back to you with that in a minute

Answer 68

Do you have the molecular mass of the hydrocarbon?

Answer 69

Not yet

Answer 70

What formula would I need to use? n=PV/RT

Answer 71

Wait no never mind

Answer 72

well to find the molecular formular we would need the vapour density of the gas
Mocelar mass = 2 * vapour density

all we have now is the formula C6H16

which might be its molecular mass

the empirical foemula could be propane C3H8
C6H16 could be a conjugate of propane

Answer 73

I need the molar mass and the empirical formula to find the molecular formula right?

Answer 74

yes

Answer 75

but we havent got molar mass

Answer 76

if it had been given in the question we could be sure

Answer 77

i would go with C3H8 for the empirical formula and C6H16 for the molecular formular

Answer 78

gotta go right now

Answer 79

Ok! Thank you!