Page 1 of 1

Error in Solutions manual

Posted: Sun Jan 20, 2019 11:49 pm
by katherinemurk 2B
I have the 6th edition and for 12.35, it says list the acids in order of increasing strength, and it says the larger the Ka value, the stronger the acid, yet the list of acids is in the wrong order and it should be the other way around. Is this an error or am I thinking about this wrong?

Re: Error in Solutions manual  [ENDORSED]

Posted: Mon Jan 21, 2019 12:20 am
by Hai-Lin Yeh 1J
There is no error in the solutions manual. If you have calculated correctly, the Ka values for the acids should be: (7.6 x 10^-3) for H3PO4, (1.0 x 10^-2) for H3PO3, (3.5 x 10^-3) for H2SeO3, and (1.2 x 10^-2) for HSeO4-. As you have stated, the larger the Ka value, the stronger the acid. This also means the smaller the Ka value, the weaker the acid.

The greatest Ka value should be: 1.2 x 10^-2. Remember that 10^-2 does NOT make the value negative. 1.2 x 10^-2 is equivalent to 0.012. If you compare just the degrees, you should notice that 10^-3 is less than 10^-2. So, only 2 numbers are 10^-2: H3PO3 and HSeO4-. 0.012 > 0.010 which means that HSeO4- is the strongest. The second strongest should therefore be H3PO3. Then, comparing the ones with the 10^-3 degrees, (7.6 x 10^-3) > (3.5 x 10^-3) so H3PO4 is the third strongest and H2SeO3 is the weakest.

H2SeO3 < H3PO4 < H3PO3 < HSeO4-

You can also try the other way where the smaller the Ka value, the weaker the acid and go from there, but ultimately, there is no error in the solutions manual.