Question

What is the pH of a solution for which [OH-] equals 5.17 x 10^-10 M?

Answer 1

We have to remember two things for this kind of problems

1 pH meaning (it is just a formula) ph is negative logarithm of the concentration of protons (H+) in Molar units

$pH=-\log \lbrack H^+\rbrack$

The secong one is that the concentration of protons [H+] times the concentration of inos OH- [OH-] in aquous solution is always 10 to the minus 14:

$\lbrack H^+\rbrack\lbrack OH^-\rbrack=10^{-14\text{ }}$

then if they give the concentration of ions OH- we can calcuate the concentration of protons and viceversa:

$\lbrack H^+\rbrack=\frac{10^{-14}}{5.17\times10^{-10}}=1.93\times10^{-5}M$

now we have the concentration of protons que only have to apply the pH formula:

$pH=-\log \lbrack H^+\rbrack=-\log (1.93\times10^{-5})=4.7$