To determine the pKa of ethylamine C2H5NH2 , we first need to find the Ka value. We can do this by using the relationship between pH and pOH, and then using the Kb value for ethylamine to find the Ka value.1. Determine the pOH of the solution:pOH = 14 - pHpOH = 14 - 9.0 = 5.02. Calculate the Kb value for ethylamine:Kb = 10^-pOH Kb = 10^-5.0 = 1.0 x 10^-5 3. Determine the Ka value using the relationship between Ka, Kb, and Kw the ion product of water :Ka * Kb = KwKa = Kw / KbKa = 1.0 x 10^-14 / 1.0 x 10^-5 Ka = 1.0 x 10^-9 4. Calculate the pKa value:pKa = -log Ka pKa = -log 1.0 x 10^-9 pKa = 9.0So, the pKa of the primary amine ethylamine is 9.0.