Ant venom contains formic acid (HCOOH; “formica” is the Latin word for ant). Suppose you are at a pharmaceutical company working on a quick antidote and need to estimate the pH at the stoichiometric point when titrating a solution of formic acid. Estimate the pH at the stoichiometric point of the titration of 25.00 mL of 0.100 m HCOOH(aq) with 0.150 m NaOH(aq).

ANTICIPATE The formate ion produced in this titration is a base, so you should expect pH > 7.

PLAN To calculate the pH at the stoichiometric point, proceed as in Examples 6D.4 or 6D.5. Note that the amount of salt at the stoichiometric point is equal to the amount of titrant added and that the volume is the total volume of the combined analyte and titrant solutions. The K_b of a weak base is related to the K_a of its conjugate acid by K_a * K_b = K_w; the K_a is listed in Table 6C.1.

What should you assume? Assume that the autoprotolysis of water has no significant effect on the pH and that because the formate ion is a very weak base, its concentration changes insignificantly when it is protonated by water.

Examples 6D.4

You are working in the emergency room of a hospital where a patient suffering from influenza has developed metabolic alkalosis, a condition in which the pH of the blood is too high. You have available a stock solution of ammonium chloride, which is used to lower the pH of the blood of patients suffering from alkalosis, but you need to know its pH. Estimate the pH of 0.15 m NH₄Cl(aq) at 25°C.

ANTICIPATE Because NH₄⁺ is a weak acid and Cl^– is neutral, you should expect pH

PLAN Treat the solution as that of a weak acid, using an equilibrium table as in Toolbox 6D.1 to calculate the composition and hence the pH. First, write the chemical equation for proton transfer to water and the expression for K_a. Obtain the value of Ka from K_b for the conjugate base by using Eq. 4a in Topic 6C (Ka = K_w/K_b). The initial concentration of the acidic cation is equal to the concentration of the cation that the salt would produce if it retained all its acidic protons.

What should you assume? Assume that (1) the extent of deprotonation is so small that the change in concentration of NH₄⁺ is insignificant, and (2) the autoprotolysis of water does not affect the pH significantly. Check these assumptions at the end  of the calculation.

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TABLE 6C.1 Acidity Constants at 25 °C* K₂ 3.0 × 10-1 2.0 × 10-1 Acid trichloroacetic acid, CCI,COOH benzene sulfonic acid, C,H,SO3H iodic acid, HIO3 sulfurous acid, H₂SO3 chlorous acid, HClO₂ phosphoric acid, H3PO4 chloroacetic acid, CH₂CICOOH lactic acid, CH3CH(OH)COOH nitrous acid, HNO₂ hydrofluoric acid, HF 1.7 X 10¹ 1.5 × 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 104 pKa 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, C,H₂COOH acetic acid, CH3COOH carbonic acid, H₂CO3 hypochlorous acid, HClO hypobromous acid, HBrO boric acid, B(OH)3¹ hydrocyanic acid, HCN phenol, C,H,OH hypoiodous acid, HIO Ka 1.8 x 10-4 6.5 x 10-5 1.8 X 10 4.3 X 10 3.0 × 10-8 2.0 × 10-⁹ 7.2 X 10-10 4.9 × 10-10 1.3 X 10-10 2.3 × 10-11 PK₁ 3.75 4.19 4.75 6.37 7.53 8.69 9.14 9.31 9.89 10.64