[mattg2006]

I have this question:

An alcohol has a relative molecular mass of 74.0 and has the following composition by mass: C, 64.9%; H, 13.5%; O, 21.6%

Calculate the empirical formula of the alcohol and show that its molecular formula is the same as the empirical formula.

This is what I've done so far:

C 74 x 0.649 = 48.026
H 74 x 0.135 = 9.99
0 74 x 0.216 = 15.984

This is roughly C48 H10 and 016 (all add up to 74)

I have then divided each one by its mass.

C 48/12 = 4
H 10/1 = 10
O 16/16 = 1

I know the structure of an alcohol is C_nH_2n+1OH

There the empirical formula is: C₄H₉OH

First of all, is this correct?

How do I show its molecular formula is the same? (How do I calculate molecular formula?)

Thanks,

[Unco]

G'day, mattg2006.

Recheck with your textbook about how this is done.

Say we have 100g of this substance. So we have 64.9g of C, 13.5g of H and 21.6g of O.

We need to convert those masses to amounts (in moles), by n = m/M. The molar ratio determines the empirical formula.

Once you have C, H and O in moles, determine which has the least number of moles. Divide the other two's number of moles by this to get two molar ratios of each of  the two higher-amount elements to the lowest-amount one, and hence the empirical formula.

The final step is to check this empirical formula gives a molecular mass equal to that stated in the question. If it is half the actual molecular mass, the ratio must be doubled, for example.

[mattg2006]

I've had another go and came up with the same answer! Where am I going wrong?

First of all, with that question, should I presume 100g of substance?

If so,

C - 64.9g, H - 13.5g, 0 - 21.6g

Moles = Mass/Molar mass

C - 64.9/12 = 5.408333333
H - 13.5g/1 = 13.5
O - 21.6g/16 = 1.35

Oxygen has the least number amount of moles.

So,

5.40833333/1.35 = 4.00617, roughly 4
13.5/1.35 = 10
1.35/1.35 = 1

This is exactly the same numbers I had before.

Where am I going wrong?

Thanks for your help Unco.

[Borek]

There the empirical formula is: C₄H₉OH

First of all, is this correct?

Seems OK with me. Both methods give the same answr.

How do I show its molecular formula is the same? (How do I calculate molecular formula?)

Molar mass is given so there is no problem with checking how many times C₄H₉OH must be repeated to obtain molceluar formula.

[mattg2006]

Doesnt it need to only be repeated once to obtain the molecular formula?

I'm slightly confused about how to even calculate the molecular formula from that.

Thanks.

[Borek]

Doesnt it need to only be repeated once to obtain the molecular formula?

Yes

I'm slightly confused about how to even calculate the molecular formula from that.

Multiply all coefficients by 1

[mattg2006]

Is it because:

4 Carbons = 4 x 12 = 48
10 Hydrogens = 10 x 1 = 10
1 Oxygen = 1 x 16 = 16

48+10+16 = 74.

Is this why?

[Borek]

Yes. You have calculated molar mass of the empirical formula - which can be repeated several times in reality. In this case it is repeated once.

[mattg2006]

Could you give an example when I would need to repeat the empirical formula?

Thanks.