Answer:1.0⋅10−7
Explanation:Start by writing a balanced chemical equation for the partial ionization of the acidHA(aq]+H2O(l]⇌A−(aq]+H3O+(aq]Notice that you have 1:1 mole ratios across the board. For every mole of acid that ionizes in aqueous solution, you get one mole of its conjugate base and one moleof hydronium ions, H3O+.In other words, the equation produces equal concentrations of conjugate base and hydronium ions. Now, you can use the pH of the solution to calculate the equilibrium concentrationof the hydronium ions. pH=−log([H3O+])⇒[H3O+]=10−pHIn your case, the pH of the solution is equal to 4, which means that you'll have[H3O+]=10−4MBy definition, the acid dissociation constant, Ka, will be equal to Ka=[A−]⋅[H3O+][HA]The expression for the acid dissociation constant is written using equilibrium concentrations. So, if the reaction produced a concentration of hydronium ions equal to 10−4M, it follows that it also produced a concentration of conjugate base equal to 10−4M.Because the initial concentration of the acid is considerably higher than the concentrations of the conjugate base and hydronium ions, you can approximate it to be constant. This means that the acid dissociation constant for this acid will be Ka=10−4⋅10−40.100=1.0⋅10−7This is the underlying concept behind an ICE table HA(aq]+H2O(l] ⇌ A−(aq] + H3O+(aq]I 0.100 0 0C (−x) (+x) (+x)E 0.100−x x xHere x represents the equilibrium concentration for the conjugate acid and hydronium ions. Since you know that x=10−4, you will haveKa=10−4⋅10−40.100−10−4Once again, you can use0.100−10−4=0.0999≈0.100to getKa=10−80.100=1.0⋅10−7