# Chemical Forums

## Chemistry Forums for Students => High School Chemistry Forum => Chemistry Olympiad and other competitions => Topic started by: Radu on March 25, 2014, 03:41:40 PM

Title: The first problem
Post by: Radu on March 25, 2014, 03:41:40 PM
http://icho2014.hus.edu.vn/Preparatory-Problems/0001/01/Preparatory-Problems-224.html

I am having  a problem with the first problem, the last question. I obtain a value of 101.75° for C-O-H angle in methanol, when it should be clearly between 105°(water) and 110°(diethylether), it actually is 108°.
Here's my work:
Applying a consequence of  cosinus theorem:
μwater=sqrt2*μO-H*sqrt(1+cos105°). Hence, μO-H=1.5158 D .
Analogously:  μdiethylether=sqrt2*μO-C*sqrt(1+cos110°). Whence,
μO-C=1.12788 D .
Now, for methanol: α=(μ2methanol2O-C2O-H)/2μC-OμO-H.
Is there any mistake? Maybe they neglected C-H interactions, even if they are more or less included in experimental C-O dipole moment from diethylether.

Title: Re: The first problem
Post by: Rutherford on March 25, 2014, 04:06:02 PM
I got the same. It's probably because of the neglected C-H interaction.