Answer:- 3) [tex]C_nH_2_n[/tex]
Explanations:- [tex]C_nH_2_n_-_6[/tex] is not a valid formula for an alkane, alkene or alkyne.
[tex]C_nH_2_n_-_2[/tex] 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 [tex]C_2H_2[/tex] .
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 [tex]C_nH_2_n_-_2[/tex] then:
[tex]C_2H_4_-_2[/tex]
= [tex]C_2H_2[/tex]
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, [tex]C_nH_2_n[/tex] then:
[tex]C_2H_4[/tex]
It could be written as:
[tex]H_2C=CH_2[/tex]
There is a has a double bond between the carbon atoms and two H are bonded to each carbon atom so, [tex]C_nH_2_n[/tex] is the right formula of an alkene.
If we plug in n = 2 in the fourth formula then:
[tex]C_2H_4_+_2[/tex]
= [tex]C_2H_6[/tex]
It could be written as [tex]H_3C-CH_3[/tex] , it has a single bond between carbons and so it is an alkane.
