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# Molar ratio Watch

1. I solved the question algebraically but the method outlined in the mark scheme, which uses a molar ratio, seems much simpler
However, i dont understand it.
How would you answer this question?
Thanks
2. I would do it algebraically

The other way to do it is to say 16.3% by mass is 16.3/18 and 83.7/460.5 (the mass compound without water)

gives ratios 0.9055 and 0.1817, division by the smallest => x = 5

it's very much like the question 'a hydrocarbon is 53.5% carbon, 9% hydrogen the remainer oxygen calculate the empirical formula' except you're dividing by molecules rather than atoms

edit: typo!
3. (Original post by EierVonSatan)
I would do it algebraically

The other way to do it is to say 16.3% by mass is 16.3/18 and 83.7/460.5 (the mass compound without water)

gives ratios 0.9055 and 0.1817, division by the smallest => x = 5

it's very much like the question 'a hydrocarbon is 53.5% carbon, 9% hydrogen the remainer oxygen calculate the empirical formula' except you're dividing by molecules rather than atoms

edit: typo!

Ah. The 'other way' method is assuming that you have 100g of the chemical right?

So then n =m/Mr can be applied, giving you the molar ratio.
4. (Original post by sonic23)
Ah. The 'other way' method is assuming that you have 100g of the chemical right?

So then n =m/Mr can be applied, giving you the molar ratio.
No you don't need to assume a certain mass - you have the percentages.

Methane is 75% carbon by mass, 25% hydrogen

75/12 = 6.25 and 25/1 = 25

25/6.25 = 4

CH4

Updated: June 14, 2009
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