Question

What is the molarity of 555 L of a Ba(OH)2 solution if the pH is 10.20?

Answer 1

pH is a symbolic representation of:

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

and for water we now that:

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

that means the text give us the information to calculate the concentration of protons and therefore the concentration of ions OH-:

$pH=10.2\rightarrow\lbrack H^+\rbrack=10^{-10.2}^{}$

therefore the concentration of OH- can be calculated:

$\lbrack OH^-\rbrack=\frac{10^{-14}}{10^{-10.2}}=10^{-3.8}=1.58\times10^{-4}\text{ M}$

but they are not asking about the concetration of OH- they ask about concentration of Ba(OH)2 and for that we need to know the disolution stechiometry

$Ba(OH)_2\rightarrow Ba^++2\times OH^-$

Whic mean the concentration of Ba(OH)2 is half the concentration of OH-:

$\lbrack Ba(OH)_2\rbrack=\frac{\lbrack OH^-\rbrack}{2}=7.92\times10^{-5}M$