Example #1: A student prepared a solution of salicylic acid (a monoprotic weak acid, MW = 138.123 g mol¯ 1) and measured the pH of the solution to be 2.430. Calculate the K a of a weak acid given the pH and other concentration data (not molarity) Return to Acid Base Menu. Since x = [H3O +] and you know the pH of the solution, you can write x = 10-2.4. Calculate the pH value from the Ka by using the Ka to find the concentrations, or molarity, of the products and reactants when an acid or base is in an aqueous solution. Hence the concentration of hydrogen ions in such solutions can be taken to be equal to the concentration of the acid. References. Calculate the K b given the pH and molarity. Return to a listing of many types of acid base problems and their solutions. Calculate K a from pH and Molarity. More on this below. Ka = (10-2.4) 2 /(0.9 - 10-2.4) = 1.8 x 10-5. You start by using the pH of the solution to determine the concentration of the hydronium ions, H_3O^(+). Strong acids and bases are compounds that, for practical purposes, completely dissociate into their ions in water. For "strong" acids and bases it is rather easy. The calculation of pH using molar concentration is different in case of strong acid/base and weak acid/base. Note that x is Related to pH and Calculate Ka; As noted above, [H3O+] = 10-pH. Calculate the pH by taking the -log of the concentration of the H3O. Create an ICE table. The salt of a weak base behaves in solution as a weak acid, and its pH is calculated as for any other weak acid, eg if the acid is monobasic then [H+] = square root of (Ka x c), in this case if pKa = 10.4, Ka = 3.98 x 10^-11, [H+] = square root of(3.98 x 10^-11 x 0.13) = 2.27 x 10^-6M, pH = 5.64 This is a favorite problem for teachers to test! Return to the Acid Base menu. It can convert pH to H +, as well as calculate pH from the ionization constant and concentration. Return to a listing of many types of acid base problems and their solutions . This is only possible with acids and bases. Here's the problem to be discussed: Problem #1: Codeine (C 18 H 21 NO 3) is a weak organic base. For "weak" acids and bases this is a little complicated, I will of course explain it if you request that (comment me for that). With this pH calculator, you can determine the pH of a solution in a few ways. Strong acid/base. It is now possible to find a numerical value for Ka.