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Practice Problems — Calculating pH given Ka and [HA]initial 1. Methanoic acid is the simplest carboxylic acid and is found naturally in the venom of bee and ant stings. (It is also called formic acid, from the Latin word for ant, formica). Calculate the pH of a 0.50 M solution of methanoic acid.
2. Household vinegar is an aqueous solution of acetic acid. A sample of vinegar is analyzed by titration and found to be 0.850 M CH3COOH. Calculate the pH of this solution.
3. Calculate the percent ionization in a 0.10 M solution of aspirin with a pH of 2.27. Would our simplifying assumption stated above be valid in this case?
Problem Type 2: Calculating [HA]initial given Ka and pH
In this type of problem, we are given the pH of the solution from which we calculate the equilibrium concentration of hydronium ions. We then use that value and the Ka to calculate the initial concentration of the weak acid. As we are once again solving for an unknown on the ICE table, we still must define x as that unknown. However, there are a couple of differences between this unknown and the one in the previous problem type: 1. We place this “x” in the “Initial” row, rather than the “Equilibrium” row of the table. 2. This unknown will be the largest of any quantity appearing in the ICE table because it’s the initial concentration of the acid. Therefore, no assumption regarding its insignificance applies. Consider the sample problem below. Note that the use of the quadratic isn’t necessary to solve this problem. Therefore, there is no need to assume that the amount of ionization of the benzoic acid that occurs relative to the initial concentration is insignificant. However, if the [H3O+]eq is sufficiently large relative to the Ka value, the assumption may be valid. Even so, until you have acquired ample experience solving these types of problems, if you don’t have to make such an assumption to solve the problem, then don’t. (Calculate the percentage ionization in the sample problem to see if such an assumption would have been valid in this case.)
238 Chapter 4 Acid-Base Equilibrium
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