# Polystyrene repeat unit molecular weight.

• Chemistry
AndrewDe
Edit: thread moved from non HW forum.

I'm trying to calculate the unit molecular weight of polystyrene. I'm given the structure, and I have to count all the elements in the structure. To calculate the weight, I keep counting 8C & 7H. The correct answer is 8C +8H. I'm having a hard time deterring where the additional H is coming from.

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Borek
Mentor
Can you draw the unit and post it as a picture?

AndrewDe
Picture from the text book.

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AndrewDe
Can you draw the unit and post it as a picture?
Posted a picture from the text book. Sorry about the wrong thread posting.

Borek
Mentor
How many hydrogen atoms on the ring? How many hydrogen atoms on the side chain?

AndrewDe
How many hydrogen atoms on the ring? How many hydrogen atoms on the side chain?
I see 4 in the ring, and 3 in the structure. Idk where I'm leaving the other H atom off.

epenguin
Homework Helper
Gold Member
There are 5 in the ring, one on each C except the C joined to the rest of unit..

Borek
Mentor
I see 4 in the ring

And - as epenguin said - that's where you are wrong. You were probably mislead by the picture and the way it depicts the hydrogen atoms on the alkane chain.

Imagine just benzene - how many hydrogens on the ring? How many hydrogens will be left on a mono substituted ring?

SciencewithDrJ
Edit: thread moved from non HW forum.

I'm trying to calculate the unit molecular weight of polystyrene. I'm given the structure, and I have to count all the elements in the structure. To calculate the weight, I keep counting 8C & 7H. The correct answer is 8C +8H. I'm having a hard time deterring where the additional H is coming from.

The benzene ring has 6 carbons and 6 hydrogens. There side chain takes the place of only of the hydrogens on the ring, leaving only 5 hydrogens on the ring. The side chain has 3 hydrogens. So, add 3 hydrogens from the side chain + 5 hydrogens from the benzene ring = total 8 hydrogens.