# How do you calculate the molar concentration of OH in a 0.075 M solution of ethylamine?

$\text{Ethylamine}$ acts as a Bronsted base in aqueous solution:
$E t N {H}_{2} \left(a q\right) + {H}_{2} O \left(l\right) r i g h t \le f t h a r p \infty n s E t N {H}_{3}^{+} + H {O}^{-}$
K_b=([EtNH_3^+][""^(-)OH])/([EtNH_2])
We could solve this equation had we a value for ${K}_{b}$ for $\text{ethylamine}$, or ${K}_{a}$ for $\text{ethyl ammonium cation}$; such values are available, and it should have been supplied with the question.