[ifrah]

are hydrocarbons containing two or more triple bonds also alkynes ?if so then why is the general formula for alkynes not applicable on them i.e CnH2n-2

[cth]

I think you shouldn't apply the "general alkyne formula" in the case of multiple triple bonds. Look at it this way:
* For alkane, the formula is C_nH_2n+2
* Remove 2 hydrogen atoms to form a double bond, you get the alkene C_nH_2n
* Remove 2 more hydrogen atoms to form either a triple bond (alkyne) or 2 double bonds (di-alkene), you get the formula C_nH_2n-2
* Remove another 2 hydrogens to form either 3 double bonds (tri-alkene) or one triple + one double bonds (alken-yne), you get C_nH_2n-4
* Finally, remove 2 more hydrogens to form 2 triple bonds (di-alkyne), 1 triple and 2 double bonds (alk-dien-yne) or 4 double bonds (tetra-alkene)  C_nH_2n-6

Following this reasoning, for 3 triple bonds you have C_nH_2n-10.

It is better to understand where this kind of formula come from, so you don't try to use them for cases where they don't apply.

[darkness1102]

Remember the general formula for hidrocacbons is C_nH_2n+2-2k and k is number of double bond.

[ifrah]

if thats the general formula for hydrocarbons where k is no of double bonds then what about triple bonds?
also
where would u place 1,5_hexadiyne?

[cth]

For a non-cyclic compound, you would have:

C_nH_2n+2-2k-4m
k: number of double bonds
m: number of triple bonds

Or, you can consider the triple bond as being equivalent to two double bonds.

But, I think it is much better to understand how to find this relation when needed, rather than learning it by heart. If you don't understand it, you run the risk to ill-apply it and get a mistake.