Answer:- 3) \(C_nH_2_n\)
Explanations:- \(C_nH_2_n_-_6\) is not a valid formula for an alkane, alkene or alkyne.
\(C_nH_2_n_-_2\) is the general formula for an alkyne that has a triple bond between carbon atoms. The example for an alkyne is acetylene and it's formula is \(C_2H_2\) .
In these general formulas, n is an integer number and for alkene and alkyne the minimum value of n is 2.
Acetylene has two carbons so the value of n for it is 2. If we plug in the value of n in the formula \(C_nH_2_n_-_2\) then:
\(C_2H_4_-_2\)
= \(C_2H_2\)
In acetylene, there is a triple bond between the carbon atoms and each hydrogen is bonded to each carbon.
If we plug in n = 2 in the formula, \(C_nH_2_n\) then:
\(C_2H_4\)
It could be written as:
\(H_2C=CH_2\)
There is a has a double bond between the carbon atoms and two H are bonded to each carbon atom so, \(C_nH_2_n\) is the right formula of an alkene.
If we plug in n = 2 in the fourth formula then:
\(C_2H_4_+_2\)
= \(C_2H_6\)
It could be written as \(H_3C-CH_3\) , it has a single bond between carbons and so it is an alkane.
