We have seen that the formula for calculating generated resulting from a weak base is: Simplified treatment of polyprotic acid solutions The calculations shown in this section are all you need for the polyprotic acid problems encountered in most first-year college chemistry courses. So we have: Reacting acetic acid with aqueous ammonia produces ammonium acetate and water.
Let's do another one. XP, Vista, 7, 8, 10 single user license price: Compare the successive pK a values of sulfuric and oxalic acids see their structures in the box, above right , and explain why they should be so different.
Substitution into the equilibrium expression yields Can we simplify this by applying the approximation 0. Practice Work the problems at the link below: Calculate the pH of the solution. So we're talking about ammonium acting as an acid here, and so we're gonna write an equilibrium expression.
Doing so yields.
Better to avoid quadratics altogether if at all possible! The pH of a solution of ammonium chloride can be found in a very similar way to the sodium fluoride solution in Sample Problem 21. Step 3: That yields an important approximation:. We will find the pH of a 2.
This, of course, is a sure indication that this treatment is incomplete. Conversely, if you neutralize acetic acid a relatively weak acid with sodium hydroxide a very strong base , the resulting neutralized solution of sodium acetate will have a pH greater than 7.
However, the "weak ion" CN — , being the conjugate base of a weak acid, will have a tendency to abstract protons from water: As usual, the most general approach is unsuitable for calculations done by hand.
As we pointed out in the preceding lesson, the "effective" value of an equilibrium constant the activity will generally be different from the value given in tables in all but the most dilute ionic solutions. This salt is so symmetrical, that results of pH calculation just must be perfect:.
CC licensed content, Shared previously. Rewriting the equilibrium expression in polynomial form gives. Salts of Polyprotic Acids Do not be intimidated by the salts of polyprotic acids.