What is its molecular formula?A 2.203 sample of an organic compound was extracted from a plant. When it was burned in oxygen, the hydrogen in the compound was converted to 1.32g of water and the...

What is its molecular formula?

A 2.203 sample of an organic compound was extracted from a plant. When it was burned in oxygen, the hydrogen in the compound was converted to 1.32g of water and the carbon was oxidised to 3.23g of carbon dioxide.

Another sample was analysed in a mass spectrometer. The mass spectrum produced showed that the molar mass of the compound was 60.0g mol. What is its molecular formula?

Asked on by jesicatran

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bandmanjoe's profile pic

bandmanjoe | Middle School Teacher | (Level 2) Senior Educator

Posted on

Let's go with the sample that was analyzed in the mass spectrometer.  To calculate the molar mass of the unknown compound, start with what is already known.  We know the standard formula for organic compounds is composed of carbon and hydrogen atoms.  Carbon has a mass number of 12.011 amu, while hydrogen has 1.0079 amu for it's mass number.  The only thing we don't know is how many atoms of each for the molecular formula.

The standard formula for simple organic compounds is CxH(2x +2), meaning there are twice the number of hydrogen atoms plus two more as there are carbon atoms.  Four carbon atoms would have an amu of 48.044.  Ten hydrogen atoms would have an amu of 10.079.  If you add the two amu amounts together, you get 58.123, which is remarkably close to the 60 amu the spectrometer arrived at.  I would guess the correct molecular formula to be C4H10, butane.

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