# The pH of a soft drink is determined to be 4.0. What is the [OH^-] of the drink?

$\left[H {O}^{-}\right] = {10}^{- 10} \cdot m o l \cdot {L}^{-} 1$.
In aqueous solution, $p H + p O H = 14$
And if $p H = 4$, then necessarily $p O H = 10$, and thus $\left[H {O}^{-}\right] = {10}^{- 10} \cdot m o l \cdot {L}^{-} 1$.