Question

an unknown compound contains only c , h , and o . combustion of 3.50 g of this compound produced 6.99 g co2 and 2.86 g h2o . what is the empirical formula of the unknown compound? insert subscripts as needed.

Answer 1

The empirical formula of the unknown compound is C2H4O

Number of moles = mass / molar mass

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molar mass of CO2 = 44 g/ mole

Moles of CO2 =6.99/44 = 0.152 moles

Since there is 1 mole of C in CO2  

so, no. of moles of C = 0.152moles

mass of C = 0.152x 12 = 1.824  g

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molar mass of H2O = 18 g/ mole

moles of H2O =2.86/18 = 0.158moles

Since there are 2 moles of H in H2O

so, no. of moles of H = 0.158x2 = 0.317  moles

mass of H = 0.317x1.0079 = 0.3202g

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Total mass of H and C = 0.3202+1.824   = 2.144g

mass of sample =  3.50 g

mass of O =  3.50 - 2.144= 1.356 g

moles of O=1.356/16= 0.08475 moles

molar ratio of C : H : O =1.824  : 0.3202 : 0.08475

smallest number  = 0.0847

Divide number of moles of C,H and O by the smallest number:

Molar ratio of C : H : O = 21.53 : 3.9 : 1.0

                                        = 2:4:1

Hence the Empirical formula is C2H4O

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