>> The number of covalent bonds formed by carbon is 90% <<
Not sure where the "90%" comes from, but carbon forms four covalent bonds. In any combination of four total bonds (four single, two single & one double, or one single & one triple) the carbon atom has a zero formal charge (let me know if you need a definition of that!).
>> Hydrocarbons are molecular compound containing only carbon and hydrogen atoms <<
That's correct. 
>> Give the general formulas for alkanes, alkenes, and alkynes. <<
Alkanes: CnH2n+2
Alkenes: CnH2n
Alkynes: CnH2n�?
Correct.
>> Name the following alkanes. <<
1) CH3-CH2-CH2-CH2-CH-CH3
|
CH3
1A = 2-Methyl Hexane
That's correct, 2-methylhexane.
2) CH3-CH2-CH2-CH-CH3
|
CH2
|
CH3
2A = 1-ethyl pentane
Oops, first check the structure; the "ethyl group" should be on the CH carbon, second from the end, not on the end CH3 carbon, Otherwise that CH3 carbon will have five bonds on it, a no-no. Here's what it should be, with the main chain highlighted:
CH3-CH2-CH2-CH-CH3
|
CH2
|
CH3
Remember Rule 1, finding the longest carbon chain; this is 3-methylhexane. Going back to the structure you first had, but correcting the hydrogens,
CH3-CH2-CH2-CH2-CH2
|
CH2
|
CH3
this is simply heptane, CH3-CH2-CH2-CH2-CH2-CH2-CH3. Don't be fooled by a crooked main chain, it does not have to be drawn as a straight horizontal chain, although that is usually the best. 
Steve
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