Help determining empirical/molecular formula from given mass data

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Having a lot of trouble getting started. The question is:



The combustion of 40.5 mg of a compound containing C, H, and O produces 110 mg of CO2 and 22.5 mg of H20. The molar mass of the compound is 162 g/mol. Determine the empirical and molecular formulas.

I don't know how to approach this problem because I've usually been given the mass percents of each element in the unknown compound. Not sure how to use what I've been given to determine the formulas.

Oct 3rd, 2015

Thank you for the opportunity to help you with your question!

number of moles = mass / molar mass
molar mass of CO2 = 44 g/ mole
0.110 g of CO2 has 0.110 /44 = 0.00250 moles of CO2
there is 1 mole of C in CO2 and all the C from the compound becomes CO2
so moles of C in the compound = 0.00250 moles


molar mass of H2O = 18 g/ mole
0.0225 g of H2O has 0.0225 / 18 = 0.00125 moles of H2O
there are 2 moles of H in H2O so moles of H in the compound = 0.00250 moles

mass of H + C = 0.03256 g
mass of sample = 0.0405 g
mass of O by difference = 0.00794 g
moles of O = 0.00794 /16 = 0.00050 moles

molar ratio of C : H : O = 0.00250 : 0.00250 : 0.00050
smallest number 0.00050
divide the ratio by the smallest number we get
molar ratio of C : H : O = 5.0 : 5.0 : 1.0

empirical formula is C5H5O
empirical formula mass = 5x12+5+16= 81 g
which is 1/2 the molar mass of 162 g / mol
so the molecular formula is 2 x the empirical formula
or C10H10O2

Let me know incase you need any further help ! Thanks :)
Sep 18th, 2015

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