Polystyrene repeat unit molecular weight.

[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.

 

[jim mcnamara]

https://en.wikipedia.org/wiki/Polystyrene

Look at the "wire" graphic of the styrene monomer. I get 8C and 8H.

 

[Borek]

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

 

[AndrewDe]

Picture from the textbook.

 

[AndrewDe]

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

Posted a picture from the textbook. Sorry about the wrong thread posting.

 

[Borek]

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]

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

 

[Borek]

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.