1.5 (c)

[SaniShabgahi3N]

How can you tell that the three lines are an alkyne and that the molecular formula is C3H4? I don't physically see this

[Jenna_Hakel_2A]

Any of the organic molecules that have a carbon-carbon triple bond are alkynes (just as how all single bonds is an alkane, and a double bond makes it an alkene). In all of the line diagrams, none of the carbons or hydrogens are drawn, just the bonds between carbons. Therefore, we know that the point at which the line changes from one line to three lines is the location of a carbon, which has a single bond on the left side and a triple bond on the right side. So, the carbon structure would be 3 carbons, the first two attached with a single bond and the second group of two attached with a triple bond, producing the line structure seen in the problem. From here, you can determine how many hydrogens are necessary on each carbon in order to give it 4 bonds. The central carbon, with a single bond and a triple bond, already has four bonds and requires no hydrogens. The first carbon only has one bond, so it needs 3 hydrogens. And the last carbon has a triple bond, so it needs 1 hydrogen. This gives us a total of 3 carbons and 4 hydrogens, producing the molecular formula C3H4. Hope that helped!

[Chem_Mod]

The three lines is representative of a triple bond, just like two lines are representative of a double bond. Once you know its a triple bond, you can draw out the structure, and draw in the hydrogens, this will give you four hydrogens.

[Laura_funes_1j]

I'm not quite sure what example you are talking about but you can tell it is an alkyne because of the triple bond. Also, you can tell that it is an alkyne because of the molecular formula C(n)H(2n-2)
Are you talking about the structure? If so, you would have something like
C
/// \
H-C C-H-H-H

(The format will probably get messed up but hopefully you get it.)