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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:
                       μ_water=sqrt2*μ_O-H*sqrt(1+cos105°). Hence, μ_O-H=1.5158 D .
                       Analogously:  μ_diethylether=sqrt2*μ_O-C*sqrt(1+cos110°). Whence,
     μ_O-C=1.12788 D .
                 Now, for methanol: α=(μ²_methanol-μ²_O-C-μ²_O-H)/2μ_C-Oμ_O-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.