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Moles

The Molecular Formula of Compounds.

Example 1.

A compound was found to contain
3·0 g  of carbon and 0·5 g  of hydrogen.
The relative molecular mass of the compound is 42.
What is the molecular formula of the compound?

Method.
1) Find how many moles of carbon
react with how many moles of hydrogen.

RAM of C = 12,  RAM of H = 1.

moles = mass ÷ RAM

for carbon   moles = 3 ÷ 12
                                       = 0·25 moles.

for hydrogen   moles = 0·5 ÷ 1
                                        = 0·5 moles.

2) The proportion of moles of carbon to moles of hydrogen
is reduced to the lowest whole number.

0·25 moles of C to 0·5 moles of H.
Multiply by 4.
1 mole of C to 2 moles of H.
The empirical formula is CH₂.

3) Divide the relative molecular mass of the compound by
the relative molecular mass of the empirical formula.

RMM of the compound = 42.
RMM of the empirical formula = RMM of CH₂
            = 12 + (2 x 1)
= 14.
42 ÷ 14 = 3.               

There are 3 CH₂ units in the compound.
The molecular formula of the compound is C₃H₆.
The compound is propene.

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