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)