A sample was analysed in a mass spectrometer. The mass spectrum produced showed that the molar mass of the compound was 60.0 g mol^-1. What is its molecular formula?A 2.203 g sample of an organic...

A sample was analysed in a mass spectrometer. The mass spectrum produced showed that the molar mass of the compound was 60.0 g mol^-1. What is its molecular formula?

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

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txmedteach | High School Teacher | (Level 3) Associate Educator

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This question is very similar to a question already asked:

http://www.enotes.com/science/q-and-a/find-empirical-formula-compound-308812

The method I explained here shows how you can find that the empirical formula of the organic compound is CH(2)O.

Now that we know that the molar ratio of C:H:O is 1:2:1, we can figure out the chemical formula because we know the molar mass.

Let's just let "x" be the number of oxygens per molecule. Based on the molar ratios, we can say there are then "x" carbons per molecule and "2x" hydrogens per molecule.

To determine how this relates to the molar mass, we can see that in one mole of the molecule, there are x moles of oxygen and carbon and 2x moles of hydrogen. Therefore, we can set up the following relation (MM = Molar Mass, O = Oxygen, H = Hydrogen, C = Carbon):

MM of Compound = x*MM of O + 2x*MM of H + x*MM of C

Now, we can substitute the molar masses with their actual values. We know the molar mass of the compound from the mass spectrometer result, and we can find the other molar masses by looking at the elements' mass numbers on the periodic table (you can also find the MM's through the sources below). These substitutions give us the following result:

60.0 = x*16.0 + 2x*1.01 + x*12.0

Simplifying, we get this:

60.0 = 30.02x

Dividing both sides by 30.02, we get:

x = 2.00

This means in each molecule, we have 2 atoms of oxygen, and, by the same extensions we mentioned above, 2 atoms of carbon and 4 atoms of hydrogen. This result gives us the following chemical formula:

C2H4O2

 

Sources:

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