[Rutherford]

Why is the left molecule optical active and the right one isn't?
http://imageshack.us/photo/my-images/651/optd.gif/

[Babcock_Hall]

What property does the molecule on the right have that renders it optically inactive?

[Dan]

Question is flawed - ribaric acid and erythitol are both optically inactive (meso compounds).

[Rutherford]

Question is flawed - ribaric acid and erythitol are both optically inactive (meso compounds).

Wasn't written so in my book, so I was confused.
Thanks for the help.

[Dan]

Question is flawed - ribaric acid and erythitol are both optically inactive (meso compounds).

Wasn't written so in my book, so I was confused.
Thanks for the help.

Double check the book to make sure both termini are actually the same (COOH) - if the termini are not identical then it will be chiral, e.g. ribonic acid and ribose are chiral. Assuming you've copied the structure out of the book correctly, there is a mistake in the book.

Can I ask which text it is?

[Rutherford]

Completely the same as in the book.
It isn't a famous book and it isn't the first mistake in it.