Aryanajigar
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An organic compound X which contains carbon , hydrogen and oxygen only has an mr of 58. When 0.43g of X are burned in excess oxygen , 1.1 g of CO2 and 0.45g of H2O are formed. Find the empirical and molecular formula of compound X
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Adambrl
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Alright, I've cracked it with the help of one of my classmates.

Firstly, write down the equation:

X + O2 --> CO2 + H2O

then find out the moles of X, CO2 and H2O by the masses given and their respective Mr:

n(x) = 0.43/58 = 0.00741379 mol
n(CO2) = 1.1/44 = 0.025 mol
n(H2O) = 0.45/18 = 0.025 mol

put these molar quantities into the simplest whole number ratio by dividing each individually by the smallest value:

which gives a ratio of 1 : 3 : 3

now assign these values to the relevant molecule and make a balanced equation:

(1)X +4O2 --> 3CO2 + 3H2O

use this information to create a molecule with an Mr of 58:

from the equation, you know it has 3 carbons and 6 hydrogen atoms, but that only adds up to 42. however, you know it has at least 1 oxygen molecule, therefore giving you an Mr of 58. this Molecule has the general formula C3H6O which is both it's molecular AND its empirical formula.

Hope It helps!
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Aryanajigar
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What you say actually makes sense like it really does but when i looked at the answers it said C5H10O for both formulas. Thank you for replying though! seeing as no one else had
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Aryanajigar
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(Original post by Adambrl)
Alright, I've cracked it with the help of one of my classmates.

Firstly, write down the equation:

X + O2 --> CO2 + H2O

then find out the moles of X, CO2 and H2O by the masses given and their respective Mr:

n(x) = 0.43/58 = 0.00741379 mol
n(CO2) = 1.1/44 = 0.025 mol
n(H2O) = 0.45/18 = 0.025 mol

put these molar quantities into the simplest whole number ratio by dividing each individually by the smallest value:

which gives a ratio of 1 : 3 : 3

now assign these values to the relevant molecule and make a balanced equation:

(1)X +4O2 --> 3CO2 + 3H2O

use this information to create a molecule with an Mr of 58:

from the equation, you know it has 3 carbons and 6 hydrogen atoms, but that only adds up to 42. however, you know it has at least 1 oxygen molecule, therefore giving you an Mr of 58. this Molecule has the general formula C3H6O which is both it's molecular AND its empirical formula.

Hope It helps!
omg I'm so stupid I'm sorry that u went to the effort of working this out with yur friend the mr is actually 85 not 58
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Adambrl
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(Original post by Aryanajigar)
What you say actually makes sense like it really does but when i looked at the answers it said C5H10O for both formulas. Thank you for replying though! seeing as no one else had
(Original post by Aryanajigar)
omg I'm so stupid I'm sorry that u went to the effort of working this out with yur friend the mr is actually 85 not 58
Haha, not a problem! The method works either way, as I just did it with the adjusted mr.

Alternatively, you could have found the moles of X, CO2 and H20 and using those numbers worked out the mass of oxygen by:

M(C) = n(CO2) x mr(C)

do the same for hydrogen, add up the masses and subtract from the initial mass of x. This gives you the mass of oxygen, which can be used to work out number of moles.

from then on you’ll get a ratio of 5 : 10 : 1 which corresponds to C5H10O.
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Aryanajigar
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(Original post by Adambrl)
Haha, not a problem! The method works either way, as I just did it with the adjusted mr.

Alternatively, you could have found the moles of X, CO2 and H20 and using those numbers worked out the mass of oxygen by:

M(C) = n(CO2) x mr(C)

do the same for hydrogen, add up the masses and subtract from the initial mass of x. This gives you the mass of oxygen, which can be used to work out number of moles.

from then on you’ll get a ratio of 5 : 10 : 1 which corresponds to C5H10O.
I did exactly that 60 + 10 =70
85-70=15 but the mass of oxygen is 16? Please explain that and sorry for asking to many questions
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Adambrl
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(Original post by Aryanajigar)
I did exactly that 60 + 10 =70
85-70=15 but the mass of oxygen is 16? Please explain that and sorry for asking to many questions
Not a problem, did you use the original method or the second one?
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Aryanajigar
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this time i used the second method. However last night i tried the first method and realized that 15 more needs to be added to get to an mr of 85. Or that the actual mr is 86 but i checked the question again and it said mr of 85
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Adambrl
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(Original post by Aryanajigar)
this time i used the second method. However last night i tried the first method and realized that 15 more needs to be added to get to an mr of 85. Or that the actual mr is 86 but i checked the question again and it said mr of 85
the second method would still give you the same answer, and you said that the answer given was the same, so it might just be a slight error in the question
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Aryanajigar
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(Original post by Adambrl)
the second method would still give you the same answer, and you said that the answer given was the same, so it might just be a slight error in the question
But then how did u get C5H10O
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Adambrl
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(Original post by Aryanajigar)
But then how did u get C5H10O
because with the empirical formula, it's the simplest whole number ratio. when you do the divisions by the smallest mole, it won't always give you a whole number, but something really close to one like for example 4.924525356352 etc which you'd round up to 5. the difference of 1 in the mr makes a really small difference either way, so it would still be rounded up to 5, thus getting the answer. did this help?
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Aryanajigar
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Yes i did thanks!
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