Isomer

What exactly didn't you understand and why do you need help to solve these questions?

To 4:
Butanal is an aldehyde. Do you know the functional group of this organic substance?

To 5:
pentanoic acid is a carboxylic acid. Can't you guess the functional group out?

To 6:
It is an alkene. Do you know the difference between an alkane and alkene? clue: it has to do with a bond. On which carbon atoms can be this bond? name the position of this bond and thus the isomers.

To 7:
That is a halogenalkane, so an alkane with chlorine on it to be exact. On which carbon atoms can this chlorine be placed?

To 8:
Halogenalkane again, but with two bromine atoms as halogen this time. How can those two be distributed on the chemical structure? on which carbon atoms they can be bonded?

would this be correct (i dont know if my drawing are right)

That’s not pentanoic acid you’ve drawn that’s propyl ethanoate which is an ester, which works as a functional group isomer. The carboxylate group is always at the end of the chain so you don’t number it’s position.

I'm confused because pentanoic acid is C5H10O2 which is what I drawer

The aldehydes in 4a) and 4c) are wrong too. There is no Aldehyde group (-CHO) drawn for this kind organic substance. And as it is written above, pentanoic acid as a carboxylic acid has a carboxyl group (-COOH), that is what can't be seen in your sketch.

The aldehydes in 4a) and 4c) are wrong too. There is no Aldehyde group (-CHO) drawn for this kind organic substance. And as it is written above, pentanoic acid as a carboxylic acid has a carboxyl group (-COOH), that is what can't be seen in your sketch.

I've drawn C double bond O single bond O and H at the end of pentanoic acid which gives me COOH? What am I doing wrong?

The functional groups are on the first C-atoms:

For the -CHO on aldehydes, an O double bond and an H are connected on this C atom.

For the -COOH in carboxylic acid, an O double bond and -O-H bond is on the first C-atom connected.

Both are not drawn in your sketches.

Like that