# Homework Help: Polystyrene repeat unit molecular weight.

1. Sep 27, 2017

### 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.

Last edited by a moderator: Sep 27, 2017
2. Sep 27, 2017

### Staff: Mentor

3. Sep 27, 2017

### Staff: Mentor

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

4. Sep 27, 2017

### AndrewDe

Picture from the text book.

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5. Sep 27, 2017

### AndrewDe

Posted a picture from the text book. Sorry about the wrong thread posting.

6. Sep 27, 2017

### Staff: Mentor

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

7. Sep 27, 2017

### AndrewDe

I see 4 in the ring, and 3 in the structure. Idk where I'm leaving the other H atom off.

8. Sep 27, 2017

### epenguin

There are 5 in the ring, one on each C except the C joined to the rest of unit..

9. Sep 28, 2017

### Staff: Mentor

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?

10. Oct 24, 2017

### SciencewithDrJ

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.