Nitrous acid, HNO2, is a weak Bronsted-Lowry acid with a Ka value of 4.43x10-4 mol dm-3. Calculate the pH of 0.375 mol dm-3 of HNO2.

We know how to calculate pH from H+ concentration with the equation : pH = -log₁₀[H+] So, we need to work out the concentration of H+ ions in 0.375 mol dm^-3 of nitrous acid, HNO₂. We're given the Ka value, so we should write out the expression for the acid dissociation constant Ka : Ka = ([H+][NO₂-])/[HNO₂] We assume that for every H+ ion there is one NO₂- ion. Therefore ([H+][NO₂-]) can be written as [H+]². Rearranging for [H+] and subbing in values we get this : [H+] = sqrt ( Ka x [HNO₂] ) = sqrt (4.43 x 10^-4 x 0.375) = 0.0128889 mol dm^-3 Finally we sub this into the equation for pH : pH = -log₁₀(0.0128889) = 1.89 (to two decimal places)