how do I find the value of -log on a scientific calculator?

Question

A 0.00730 mol sample of a weak acid, HY, is dissolved in enough water to form 0.0684 L of solution. If the pH of the solution is 2.03, what is the Ka of HY? 

I need to solve this problem but i cannot b/c I do not know how to punch that in

Marat · Accepted Answer

n(HY)= 0,00730 mole
V(HY)= 0,0684 L
pH= 2,03
Ka= ?

c(HY)= n(HY)/V(HY)= 0,00730 mole / 0,0684 L= 0,1067 mole L^-1

Ka= [H+][Y-]/[HY]     ([H+]= [Y-])

pH= -log[H+]

[H+]= 10^-pH= 10^-2.03= 0,00933 mole L^-1

Ka= (0,00933 mole L^-1 * 0,00933 mole L^-1) / 0,1067 mole L^-1= 0,000816 mole L^-1 or 8,16*10^-4 mole L^-1

;)

p.s. for more accurate result use  10^-2.03 instead of 0,00933

Anonym · Answer

Well, since you didn't ask for help to "solve" the problem, then i'll just tell you exactly what your question was directed towards.  There is no automatic -log button, you have to type in the negative sign followed by the log and that would = -log.  Have fun solving that....i think it looks like an acid-base titration, well, whatever, good luck!  Lol.

Anonym · Answer

You likely have to hit the negative sign last, as in, type log(A/B), then hit the '-' sign button.  I have a number of scientific calculators that require you to do just that... for future reference.

h e r · Answer

theres a little negative sign on the upper half of the screen that looks like this:  (-)

then hit the log button after it, and type in the number that you want to put it....

how do I find the value of -log on a scientific calculator?

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