The idea is to connect the atoms together in as many ways possible in order to get all the molecules with those formulas. You just have to be careful that you obeyed the normal bonding patterns for each atom and that no structures are repeated.
For the first one, C3H8O, you can connect the carbons and oxygens as follows:
C–C–C–O = CH3CH2CH2OH (1-propanol)
C–C–O–C = CH3CH2OCH3 (methyl ethyl ether)
C–O–C–C = CH3OCH2CH3 (methyl ethyl ether again, so this one doesn't count)
O OH
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C–C–C = CH3CHCH3 (2-propanol)
Those are condensed structural formulas on the right. Structural formulas show all of the bonds. For example,
H H H
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H–C––C––C––O–H
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H H H
Common bonding patterns:
Halogens: one bond –Cl
Oxygen: two bonds –O�?nbsp; and =O
Nitrogen: three bonds –N�?nbsp; and =N�?nbsp; and ≡N
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Carbon: four bonds –C�?nbsp; and =C and ≡C�?nbsp; and =C=
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When the lone electron pairs are added to the halogens, oxygen, and nitrogen, the octet rule is satisfied and the formal charge on the atom is zero. 
Steve
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