SCH4U1 EA11 Name: Date: Organic Acids The majority of weak acids are organic acids, meaning they are carbonbased compounds. Most contain a group known as the carboxylic acid group: acid form conjugate base form For example, benzoic acid and its conjugate base benzoate: * Note that sodium benzoate, the salt of the conjugate base of benzoic acid, will act as weak base in solution: + Na + Diprotic and Triprotic Acids These acids have more than one K value, a few are shown in the table below: a Weak Acids Weak acids do not completely ionize. Most naturally occurring acids (e.g. fatty acids, acetic acid, citric acid) and indicators are weak acids. Weak acids can be dangerous in concentrated form. For example, hydrofluoric acid is considered a weak acid but is used to etch glass. See a list of some common weak acids. Acid Equilibrium Constant ( K ) a For acetic acid: CH COOH 3 (aq) acetic acid + H O 2 (l) Ka = ⇌ + − H O + CH COO 3 (aq) 3 (aq) acetate ion [H 3O+][CH 3COO−] [CH 3COOH] + as the K ↑, [H O ] ↑ a 3 the larger the K value, the stronger the acid is. a 1) Calculation of K a + The [H O ] of a 0.0100 M solution of benzoic acid (C H COOH) is measured to be 3 6 5 0.00080 M. Calculate K for this acid. a 2) Calculation of pH for a Weak Acid 5 Given the K for benzoic acid is 7.0 x 10 , calculate the pH of a 0.33 mol/L solution of a benzoic acid. 3) Mixing Acids and Conjugate Bases + Calculate the [H O ] of a solution containing 0.10 M benzoic acid and 0.30 M sodium 3 5 benzoate (NaC H COO). K = 7.0 x 10 6 5 a
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