What is the empirical/molecular formula of a compound formed from 0.961g carbon reacts with 0.161g of hydrogen and 1.28g of oxygen (molmass is 180.16)

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ncchemist | eNotes Employee

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First we need to convert the grams of each element into moles.  We do this by dividing each mass by the atomic weight for each element.

For carbon: 0.961/12.01 = 0.080 moles C

For hydrogen: 0.161/1 = 0.161 moles H

For oxygen: 1.28/16 = 0.080 moles O

When comparing the ratios of the moles, we can see that the C:H:O ratios simplify to 1:2:1.  In other words, the carbon and oxygen moles are the same amount and the hydrogen moles are twice that amount.  This gives the empirical formula as CH2O.  In order to find the true molecular formula of the compound, we can compare the empirical formula to the molecular weight (180.16).  If we had just CH2O, the molecular weight would be about 12+2+16 = 30.  This goes into 180 six times, so we multiply the empirical formula by 6 to get the molecular formula of C6H12O6.  Incidentally, this is the formula for glucose, a common sugar.

Empirical formula: CH2O

Molecular formula: C6H12O6