Question

predict the chemical formula of butyne, having one carbon-carbon triple bond

Answer 1

The general formula for an alkane is: \(\sf \color{red}{C_nH_{2n+2}}\)
If you make a double-bond, you're subtracting 2 protons, so then you have for an alkene: \(\sf \color{blue}{C_nH_{2n-2}}\)
and if you form a triple bond, that means 4 proton are missing, and you have: \(\sf \color{orange}{C_nH_{2n-4}}\)

Answer 2

No. when you subtract 2 protons from CnH2n+2, you get CnH2n, and when
you subtract 4 protons you get CnH2n-2

Answer 3

You're absoltely right. Thank you for correcting me.