Law of multiple proportions (need method check please)

[rwooduk]

Homework Statement

Homework Equations

Law of multiple proportions

The Attempt at a Solution

Carbon first:

4.61/0.39 = 11.82 C/H

Hydrogen

0.39/4.61 = 0.08 H/C

therefore 11.82/0.08 = 139.72 ~ 140

masses of carbon that combine with hydrogen are in a 140:1 ratio

Does this look right? Or have i just done the second part?

i have no idea what I am doing

Thanks for any help.

 

[BvU]

What does this mean ? You divide C/H by H/C and get C²/H². Is that the law of multiple proportions ?

 

[rwooduk]

What does this mean ? You divide C/H by H/C and get C²/H².

This was just a method i followed from here:

http://chemistry.about.com/od/workedchemistryproblems/a/law-of-multiple-proportions-problem.htm

there are 57.1 g O and 42.9 g C. The mass of O per gram C is:

57.1 g O / 42.9 g C = 1.33 g O per g C

there are 72.7 g O and 27.3 g C. The mass of oxygen per gram of carbon is:

72.7 g O / 27.3 g C = 2.66 g O per g C

Dividing the mass O per g C of the second (larger value) compound:

2.66 / 1.33 = 2

Is this method not correct?

Is that the law of multiple proportions ?

As I understand it so far for AB and AB₂ there are small number ratios for the B of each compound. I am unsure how to apply this to the question.

Many thanks for the reply

 

[BvU]

Ha, in the example, there are two substances. In one of them, there is twice as much O per C as in the other. (so one can be CO4 and the other CO2, for example -- but CO and CO2 is more probable ) .

You only have one substance, so I'm afraid you will need atomic weights to work towards a nice ratio.

 

[rwooduk]

Ha, in the example, there are two substances. In one of them, there is twice as much O per C as in the other. (so one can be CO4 and the other CO2, for example -- but CO and CO2 is more probable ) .

ahh i see thanks

You only have one substance, so I'm afraid you will need atomic weights to work towards a nice ratio.

please could you expand on this a little?

Did the 140 I got have any relevence?

thanks again

 

[BvU]

LOMP: Law which states that when elements combine, they do so in the ratio of small whole numbers (assuming they have the same type of chemical bonds).

For two substances with different weight ratios you can divide out the atomic weights.

Molecules are formed from atoms (or other molecules) in the ratio of small numbers of constituents. The constituents are not counted in amounts of mass but in amounts of atoms (or molecules). So you have to convert the masses to number of atoms by dividing by the atomic weights.

[edit] And the 140 is C² / H² in kg²/kg², for which I can't think of an interesting physical meaning

 

[rwooduk]

For two substances with different weight ratios you can divide out the atomic weights.

Molecules are formed from atoms (or other molecules) in the ratio of small numbers of constituents. The constituents are not counted in amounts of mass but in amounts of atoms (or molecules). So you have to convert the masses to number of atoms by dividing by the atomic weights.

Ahh I see, yes, ok thanks very much for your help!

 

[BvU]

You're welcome. Interesting substance ! This ratio has puzzled mankind (well, the scientific community) for a while, until August Kekule dreamed up the solution !

 

[epenguin]

Yes, as said, for multiple proportions law you need two compounds. If as well as data in #1 you use that in #3 you can answer the question.