For each of the following, write chemical equations showing how an aqueous solution containing the indicated solutes in comparable concentrations can act as a buffer solution.
(a) HNO2 and Mg(NO2)2
(b) K2SO3 and NaHSO3
(c) HN(CH3)3Cl and N(CH3)3
(d) Na2H2P2O7 and Na3HP2O7