Suppose that 0.483 g of an unknown weak acid, HA, is dissolved in water. Titration of the solution with 0.250 m NaOH(aq) required 42.0 mL to reach the stoichiometric point. After the addition of 21.0 mL, the pH of the solution was found to be 3.75.

(a) What is the molar mass of the acid?

(b) What is the value of pK_a for the acid? Identify the acid in Table 6C.1.

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TABLE 6C.1 Acidity Constants at 25 °C* Ka 3.0 X 10-1 2.0 X 10-1 1.7 X 10-¹ Acid trichloroacetic acid, CCI,COOH benzene sulfonic acid, C,H,SO3 H iodic acid, HIO3 sulfurous acid, H₂SO3 chlorous acid, HClO₂ phosphoric acid, H3PO4 chloroacetic acid, CH₂ClCOOH lactic acid, CH3CH(OH)COOH nitrous acid, HNO₂ hydrofluoric acid, HF 1.5 x 10-2 1.0 10-2 7.6 x 10-3 1.4 x 10-3 8.4 x 10-4 4.3 x 10-4 3.5 x 10-4 pK₁ 0.52 0.70 0.77 1.81 2.00 2.12 2.85 3.08 3.37 3.45 Acid formic acid, HCOOH benzoic acid, C6H₂COOH acetic acid, CH,COOH carbonic acid, H₂CO3 hypochlorous acid, HCIO hypobromous acid, HBrO boric acid, B(OH)3¹ hydrocyanic acid, HCN phenol, C,H,OH hypoiodous acid, HIO K₁ 1.8 x 10-4 6.5 x 10-5 1.8 x 10-5 4.3 x 10-7 3.0 10-8 2.0 × 10-⁹ 7.2 x 10-10 4.9 × 10-10 1.3 × 10-10 2.3 × 10-11 pKa 3.75 4.19 4.75 6.37 7.53 8.69 9.14 9.31 9.89 10.64