What is pHpH if (i)(i) [H_3O^+]=5.6xx10^-5*mol*L^-1[H3O+]=5.6×105molL1; and (ii)(ii) if [HO^-]=10^-7*mol*L^-1[HO]=107molL1?

1 Answer
Jun 25, 2017

(i)(i) pH=4.25.................

Explanation:

By definition, pH=-log_10[H_3O^+], and thus for (i)....

pH=-log_10(5.6xx10^-5)=-(-4.25)=4.25 (if I have counted up the zeroes properly........)

But since [H_3O^+]xx[HO^-]=10^-14, we can take log_10 of both sides, and gets........

log_10[H_3O^+]+log_10[HO^-]=log_(10)10^-14=-14

Equivalently, multiplying each side by -1............

underbrace(-log_10[H_3O^+])_color(red)(pH)underbrace(-log_10[HO^-])_color(blue)(pOH)=+14

And thus our defining relationship......

color(red)(pH)+color(blue)(pOH)= 14

And so (ii) if [HO^-]=10^-7*mol*L^-1, pOH=7 (why?), and pH=7. Agreed?