SOLUTION: This question is actually an example problem (not explained) from a chemistry book, but I cannot remember my log rules. pKa = - log10 Ka If Ka = 1.8 * 10^-5 , then we should be a

Question 158134This question is from textbook Organic Chemistry
: This question is actually an example problem (not explained) from a chemistry book, but I cannot remember my log rules.
pKa = - log10 Ka
If Ka = 1.8 * 10^-5 , then we should be able to find the pKa. (The answer is given b/c it's a sample, pKa = 4.7)
Mathematically: x = - log10 (1.8 *10^-5)
I don't understand what to do with the - sign in front of the log, to solve the log, one would say 10^ (1.8 * 10^-5), but then something must happen with the negative sign to give the answer of 4.7...
Also, how do you go backwards (given pKa, solve for Ka).
Same example: 4.7 = -log10 Ka
so, 10 ^ x = 4.7 ?
Then, I am lost.
Thank you.
This question is from textbook Organic Chemistry

Answer by edjones(8007) (Show Source): You can put this solution on YOUR website!
log(1.8*10^-5)=-4.7...
-log(1.8*10^-5)=4.7...
.
-4.7=log(1.8*10^-5)
10^-4.7=10^log(1.8*10^-5)
0.0000199526...=1.8*10-5 This is not exactly correct but you get the idea.
.
Ed
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