0.500 L buffer solution contains 0.200 M weak acid HA and 0.100 M its conjugate weak base NaA (sodium A). If one wants the pH of the buffer solution to be 4.20, which one should be added into the buffer, NaOH or HCl? If the concentration of NaOH or HCl is 0.050 M, how many LITERS of NaOH or HCl should be added into the buffer solution. Given Ka for the weak acid is 5.7x10^-5.