Calculate the molarity (M) of a solution of the strong acid HBr if the hydroxide ion concentration is 5.00E-12 M.

Question

aaronq · Answer

For strong acids, we assume that there is 100% ionization, that is: $\sf HBr\rightarrow H^++Br^-$

We need to find the concentration of $[H^+]$ in the solution, which is equal to $[HBr]$

$\sf \Large K_{water}^{25^oC}=[H^+]*[OH^-]$

Since you're not told a temperature, we can assume that it's 25 Celsius, and use the value:

$\sf \Large K_{water}^{25^oC}=1.0*10^{-14}~M^2$

We were given $[OH^-]$ in the question: 

$[H^+]=\dfrac{1.0*10^{-14}}{[OH^-]}=\dfrac{1.0*10^{-14}M^2}{5.00*10^{-12}M }=0.002~M=[HBr]$

The concentration (Molality) of the acid is 0.002 M