Question

You work in a chemistry lab, and are asked to prepare 500 mL of a buffer solution with pH-3.20, The weak acid solution concentration in this buffer should be 0.250 M and salt is a solid. The following steps walk you through a step by step process of this preparation. a. Choose the proper weak acids for the buffer solution Table 1. Ionization constant Ka for some weak acids Name Hydrofluoric acid Nitrous acid Fulminic acid Acetic acid Hypochlorous acid Hypobromous acid Hydrogen cyanide Formula HF HNO2 HCNO CH3CO2H HCIO HBrO HCN Ka at 25 °C pKa-log (Ka 3.5 x 10-4 4.6 x 10 2.0 x 10-4 1.8 x 10-5 2.9 × 10-8 2.8 x 10-9 4.9 x 10-10 Calculate the pKa for each weak acid and fill in the table -3.30 Based on your pH of the buffer solution, which weak acid would you use? Please explain. oe used b. Choose the proper salt for the buffer solution. Table 2. Ionization constant Ka for some weak acids Conjugate base Formula KF NaNO2 NaCNO NaCH3CO2 NaCIO NaBrO NaCN Kb at 25 °C Give the correct formula of the conjugate base within each salt, and calculate its K, value. Based on the weak acid you selected in part a, which salt would you use?

Answer 1

Part A

the pH of a buffer is calculated using the Hendersen equation as

pH = pKa + log [conjugate base]/[acid]

and the buffers with maximum buffer capacity are the ones with pH = pKa

Thus to have better buffer capacity , we can choose 3 weak acids which have pKa values close to the required pH = 3.2 .

They are

i) HF with pKa = 3.456

2) HNO2 with pKa = 3.337

3) fulminic acid HCNO with pKa = 3.699

The most suitable being KNO2 (very close pKa to 3.20)

Part B

You work in a chemistry lab, and are asked to prepare 500 mL of a buffer solution with pH 3.20 The weak acid solution concentration in this buffer should be: 0.250 M and salt is a solid. The following steps walk you through a step by step process of this preparation. a. Choose the proper weak acids for the buffer solution Table 1. Ionization constant K, for some weak acids Name Hydrofluoric acid Nitrous acid Fulminic acid Acetic acid Hypochlorous acid Hypobromous acid Hy Formula HF HNO HCNO CH CO2H HCIO HBrO HCN K, at 25 °C 3.5 x 10-4 4.6 x 10 2.0 x 10-4 1.8 x 10 2.9× 10-8 2.8 x 10 4.9 x 10-10 anide Calculate the pK, for each weak acid and fill in the table. 3.8o Based on your pH of the buffer solution, which weak acid would you use? Please explain. b. Choose the Table 2. Ionization constant K, for some weak acids proper salt for the buffer solution. We low that kbKwKa Formula KF NaNO2 NaCNO NaCH CO NaCIO NaBrO NaCN Conjugate base K at 25 °C NO2 CN Give the correct formula of the conjugate base within each salt, and calculate its Kb valuc. Based on the weak acid you selected in part a, which salt would you use?