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30.4 mg of an organic compound known to contain C, H, and O was combusted and produced...

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ntretheway | (Level 2) Adjunct Educator

Posted September 19, 2013 at 3:26 AM via web

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30.4 mg of an organic compound known to contain C, H, and O was combusted and

produced 70.4 mg of CO2 and 14.4 mg of H2O. (a) Determine the empirical formula of

this compound. An analysis of this compound by mass spectrometry determined the

molecular weight to 152 amu (or Da). (b) What is the molecular formula for this

compound?

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justaguide | College Teacher | (Level 2) Distinguished Educator

Posted September 19, 2013 at 6:04 AM (Answer #1)

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30.4 mg of an organic compound known to contain C, H, and O was combusted and produced 70.4 mg of `CO_2` and 14.4 mg of `H_2O` .

70.4 mg of `CO_2` has `(70.4/44)*12` = 19.2 mg of carbon. 30.4 mg of the compound has 19.2 mg of carbon.

14.4 mg of `H_2O` has `(14.4/18)*2` = 1.6 mg of hydrogen. 30.4 mg of the compound has 1.6 mg of hydrogen.

The rest of the mass is made up by oxygen. 30.4 mg of the compound has 9.6 mg of oxygen.

19.2 mg of carbon is `1.6/1000` mole of carbon, 1.6 m of hydrogen is `1.6/1000` mole of hydrogen and 9.6 mg of oxygen is `0.6/1000` mole of oxygen.

The empirical formula of the compound is `C_((n*1.6)/1000)H_((n*1.6)/1000)O_((n*0.6)/1000)` . The smallest value of n that makes all the subscripts integers is n = 5 

The empirical formula of the compound is `C_8H_8O_3` .

The molecular weight of the compound is 152 a.m.u.

This gives the molecular formula of the compound as `C_8H_8O_3`

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