c) 119 d) 12.2 e) 125 17. Calculate the pH of a 02 M formic acid CICooH) solutio

Question

c) 119 d) 12.2 e) 125 17. Calculate the pH of a 02 M formic acid CICooH) solution. The pKa of formic acid is 3.75 a) 2.0 b) 2.2 c) 3.0 d) 32 e) 3.6 18. Using the Henderson-Hasselbalch equation calculate the pH of a solution containing 10 ml of 050 M acetic acid and 40 ml of 0.35 M acetate ion. The pK, of acetic acid is 4.76 a) 4.31 b) 4.7 c) 5.21 d) 4.61 e) 4.9 19. The diprotic acid H,CO, has two pK, values, 6.37 and 10.20. Ar a pH of 105, which species will be most. abundant? a) HCO, and CO, b) H,CO, and HCO, c) H,cO, and HCO, d) HCO, and CO, e) HCO, and CO, 20. The triprotic acid aspartic acid has three pK, values,2.1 (a-COOH).3.9 (RH), and 98 (a-NH,"). At a pH of 98 equal concentrations of which species will be present? a) COOH/RH/NH, and COOH/R/NH b) COOH/R/NH, and COO RINH, c) COO/R/NH,' and COO/RINH, d) coo/RH/NH,' and COO RNH e) COOH/RH/NH, and COO R/NH, 21. What is the isoelectric point (pl) of aspartic acid? Use the pk, values from question 20. a) 53 b) 3.9 c) 6.9 d) 2.1 e) 3.0

Explanation / Answer

Question 20- Polytropic acids dissociate stepwise. The first dissociation happens at highest pKa value and pKa value decreases with further dissociations.

According to Henderson-Hasselbalch equation: pH = pKa + log ([A-]/[HA])

at half equivalence point, where pH = pKa ;  [A-]= [HA]

hence at pH 9.8, COOH/RH/NH3+concentration would be equal to COOH/R-/NH2

Question 21-

The pI (isoelectric point) is given by the average of the pKas that involve the zwitterion, i.e. that give the boundaries to its existence.

isoelectric point of aspartic acid= (2.1+3.9+9.8)/3 = 5.3

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