Prelab questions thanks The effectiveness of a buffer in resisting pH changes is
ID: 510365 • Letter: P
Question
Prelab questions thanks The effectiveness of a buffer in resisting pH changes is called the buffer capacity. Consideration of Equation 5 reveals that the pH of a buffer prepared from a weak acid HA and its conjugate base A^- should be independent of their total concentration as long as the ratio [HA] to [A^-] is the same. Thus, an acetic acid-acetate buffer prepared from 0.1 mole HA and 0.1 mole A^- should have the same theoretical pH as a buffer containing 1 mole HA and 1 mole A^-. The buffer capacity of the two buffers, however, will be very different. The capacity of the 0.1 moles HA/0.1 moles A^- buffer will be overwhelmed when approximately 0.09 moles of HCl or NaOH have been added. The 1 M buffer will withstand almost 10x as much strong acid to strong base before either HA or A^- is consumed. The purpose of this advanced inquiry lab is to design an effective buffer with a specific pH value of a consumer or experimental biochemistry application. The investigation begins with an introductory activity to compare the properties of three acetate buffers containing varying ratio of HA and A^-. The results provide a model for guided-inquiry design of an experiment to prepare a desired buffer and verify its properties and performance. Five different buffer "challenges" are presented-each student group chooses one. The specifications for each buffer challenge are that the pH should be within plusminus 0.5 pH units of the desired pH, and 25 mL of the buffer should maintain the desired pH plusminus 1 after 100 mL of 0.02 M HCl or 10 mL of 0.2 M NaOH have been added. Preparation of a buffer by partial of a weak acid or a weak base offers additional opportunities for inquiry. Calculate the pH value in each of the following solutions, given their [H_3 O^+] concentrations. Write balanced chemical equations for dissociation of the following weak acids and identity their conjugate bases: phosphoric acid (H_3 PO_4), formic acid (HCO_2 H), and boric acid (H_3 BO_3). What would be the composition and pH of an ideal buffer prepared from lactic acid (CH_3 CHOHCO_2 H), where the hydrogen atom highlighted in boldface is the acidic hydrogen atom? The K_a value for lactic acid 1.38 times 10^-4. Use the buffer equation to calculate the pH of buffer solutions prepared by dissolving the following amounts of acetic acid and sodium acetate, respectively, in enough water to make 1 L of solution: 0.67 moles acetic acid and 0.33 moles of sodium acetate, and 0.33 moles acetic acid and 0.67 moles of sodium acetate.
Explanation / Answer
(1)
Useful formula,
pH = - Log[H3O+]
(a) 1.6 * 10-2 M
pH = - Log(1.6*10-2)
pH = 1.80
(b) 2.5 * 10-7
pH = - Log[2.5*10-7]
pH = 6.60
(c) 5.0 * 10-5
pH = - Log(5.0 * 10-5)
pH = 4.30
(2)
(a)
H3PO4 (aq.) + H2O (l) = H3O+ (aq.) + H2PO4- (aq.)
H2PO4- (aq.) + H2O (l) = H3O+ (aq.) + HPO42- (aq.)
HPO42- (aq.) + H2O (l) = H3O+ (aq.) + PO43- (aq.)
(b) HCOOH (aq.) + H2O (l) = H3O+ (aq.) + HCOO- (aq.)
(c) H3BO3 (aq.) + 2 H2O (l) = B(OH)4- (aq.) + H3O+ (aq.)