A hydrocarbon is burnt completely in air to form 17.6g of carbon dioxide gas and 7.2g of water.

What is the molecular formula of the hydrocarbon compound?

I don wan to know the answer, please tell me how to do it

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A hydrocarbon is burnt completely in air to form 17.6g of carbon dioxide gas and 7.2g of water.

What is the molecular formula of the hydrocarbon compound?

I don wan to know the answer, please tell me how to do it

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## Answers (2)

Mass of C in 17.6g CO2 = 12/44*17.6 = 4.8g

Mass H in 7.2g H2O = 2/18*7.2 = 0.8g

Divide each by respective atomic mass

C = 4.8/12 = 0.4

H = 0.8/1 = 0.8

Divide by smaller value

C = 0.4/0.4 = 1

H = 0.8/0.4 = 2

Empirical formula = CH2

It is not possible to give the molecular formula without knowing what the molar mass of the compound is

But as an example - if you know that the molar mass = 56g/mol , then you do :

Formula mass CH2 = 12+2*1 = 14g/formula

Compound mass = 56

Number of formula units = 56/14 = 4

Molecular formula = (CH2)4 = C4H8

There will be the same number of moles of C and H on both sides of the equation, so all the Carbon in your original hydrocarbon is now present in CO2, and all H is in the H2O.

Hydrogen

Calculate moles of H in H2O

molar mass H2O = 16.00 + (2 x 1.008) = 18.016 g/mol

molar mass H = 1.008 g/mol

moles = mass / molar mass

moles H2O = 0.002560 g / 18.016 g/mol

= 1.421 x 10^-4 moles of H2O

In ever H2O there are 2 moles of H

therefore moles of H in H2O, and thus in your original hydrocarbon = 2 x 1.421 x 10^-4

= 2.842 x 10^-4 moles of H

Carbon

Calculate moles C in CO2

molar mass CO2 = (16.00 x 2) + 12.01 = 44.01 g/mol

moles CO2 = mass / molar mass = 0.012507 g / 44.01 g/mol

= 2.842 x 10^-4 moles of CO2

Every CO2 molecule has 1 Carbon, therefore moles of C in CO2 and thus in the original hydrocarbon

= 2.842 x 10^-4 moles of C

Now work out the ratio of C : H

C : H

= 2.842 x 10^-4 : 2.842 x 10^-4

To get it into whole number divide both numbers in the ratio by the lowest number (both here are the same, but this will not always be the case

C : H

= (2.842 x 10^-4 / 2.842 x 10^-4) : (2.842 x 10^-4 / 2.842 x 10^-4)

= 1:1

So empirical formula is CH