Moles as level chemistry Watch

Luj99
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#1
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a)What is the empirical formula of a hydrocarbon,which has 81.8% carbon and 18.2% hydrogen ?
b)Find the molecular formula.Given that it's molecular mass is 44
My answer for part a)is CH2, however I don't no part b)
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Luj99
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Guys I need help really fast , thank you
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charco
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(Original post by Luj99)
a)What is the empirical formula of a hydrocarbon,which has 81.8% carbon and 18.2% hydrogen ?
b)Find the molecular formula.Given that it's molecular mass is 44
My answer for part a)is CH2, however I don't no part b)
For molecular formula remember that it is an integral number of empirical formulae.

i.e. relative mass(E.F.) x n = relative mass(Mr)

simply apply 'n' to get the moleular formula from the empirical formula.
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Luj99
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I'm sorry, I still don't get it
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Avacado
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I think you calculated the empirical formula wrong - in an empirical formula the ratios need to be a whole number.
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In this case you have:

81.8/12 = 6.817
18.2/1 = 18.2

And therefore to find the Empirical formula you would need to divide both numbers by 6.817, which you did.

6.817/6.817 = 1, which is fine as it is an integer (whole) number.
However, 18.2/6.817 gives an answer of 2.67, which is not an integer number. Therefore you need to apply trial and error in order to find a ratio where both of the numbers you get as an answer, like so:

1 * 2 = 2
2.67 * 2 = 5.34

Still not a whole number, therefore you would need to repeat the process again:

1 * 3 = 3
2.67 * 3 = 8

Now that you're at a whole number, you can say that makes the correct empirical formula, in this case C3H8.
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charco
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(Original post by Avacado)
I think you calculated the empirical formula wrong - in an empirical formula the ratios need to be a whole number.
Spoiler:
Show
In this case you have:

81.8/12 = 6.817
18.2/1 = 18.2

And therefore to find the Empirical formula you would need to divide both numbers by 6.817, which you did.

6.817/6.817 = 1, which is fine as it is an integer (whole) number.
However, 18.2/6.817 gives an answer of 2.67, which is not an integer number. Therefore you need to apply trial and error in order to find a ratio where both of the numbers you get as an answer, like so:

1 * 2 = 2
2.67 * 2 = 5.34

Still not a whole number, therefore you would need to repeat the process again:

1 * 3 = 3
2.67 * 3 = 8

Now that you're at a whole number, you can say that makes the correct empirical formula, in this case C3H8.
You don't actually have to apply trial and error if you notice that 2.67 = 8/3.

It is very useful to recognise the relationship between decimals and fractions.

0.33 = 1/3
0.67 = 2/3

etc.
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Luj99
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I have gotten C3H8 now so, how should I do part b) ?
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Maker
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You know the empirical formula is C3H8, you know the RMM is 44 and you also know that C=12 and H=1. I think its obvious what the answer to question b is.
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Luj99
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#9
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Yea I got it.Thankyou guys you have been a big help
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