# finding RMM of the compound

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#1
0.02 mol of hydrocarbon X is burnt in excess oxygen. The products of the combustion are passed thru anyhydrous calcium oxide and sodium hydroxide. The mass of anyhydrous calcium oxide and sodium hydroxide solution increased by 1.44 g and 3.52 g . Determine the empirical formula and molecular formula .
I have attached the working here. I cant understand the circled part.
How can we find the RMM of the compound just based on the empirical formula of the hydrocarbon? We can only find the RMM of the compound based on the MOLECULAR FORMULA of the compound , am I right?
So, what’s the correct method to do this question?
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#2
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#3
i mean to find the RMM of the hydrocarbon , we must get the mass of hydrocarbon (using the molecular formula , not empirical formula as in the question), then take the mass of hydrocarbon (with molecular formula) and divide it by the mol .....
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6 years ago
#4
Do you know the reactions between CaO and NaOH with H2O and CO2?

If so, you can work out the masses (and therefore mol) of H2O and CO2 produced. From those amounts you know how many mol of C and H atoms there are in the original hydrocarbon. Considering there are 0.02 mol of said hydrocarbon, you can work out how many mol of C and H atoms there are in each molecule, and hence its formula.
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#5
(Original post by Pigster)
Do you know the reactions between CaO and NaOH with H2O and CO2?

If so, you can work out the masses (and therefore mol) of H2O and CO2 produced. From those amounts you know how many mol of C and H atoms there are in the original hydrocarbon. Considering there are 0.02 mol of said hydrocarbon, you can work out how many mol of C and H atoms there are in each molecule, and hence its formula.
CaO absorb H20 while NAOH absorb CO2 ... i dun really understand what do you mean .
From the above ans , we can know that the empirical formula is CH2 as C: H is in 1:2
...I'm wondering how can we take the molar mass of the empirical formula CH2 as the mass of the hydrocarbon Let's say the ans is C4H8 , then the of mass of hydrocarbon should be 0.08(8) +0.96(4) = 4.48g
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6 years ago
#6
How many mol of H2O and CO2 are absorbed?

How many mol of H and C must there have been in 0.02 mol of the hydrocarbon?
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6 years ago
#7
You know 0.02 mol ofhydrocarbon was burnt.

This produced (from thecalculations in table) 0.08 mol carbon and 0.16 mol hydrogen.

Hence looking at themol ratios, one mole of hydrocarbon has 4 mol C (0.08/0.02) and 8 molH (0.16/0,02) = Formula of hydrocarbon C4H8
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