Include a brief (fewer than 4 sentence) explanation for each of the following th
ID: 958868 • Letter: I
Question
Include a brief (fewer than 4 sentence) explanation for each of the following that cites raw data or information gathered from plots: In both words and in terms of a mathematical equation, describe the relationship between pH and the ratio of conjugate base to acid concentration that you observed in this experiment. Compare the plots of [conj. base]/[conj. acid] vs. pH and log([conj. base]/[conj. acid]) vs. pH. Compare the addition of a strong acid to water and the addition of a strong acid to your buffer. Suggest a reason for any difference you observe. Describe the effect of diluting the acid. Upon sufficient dilution, what would you predict the final pH to be? Compare the pH values for your most diluted buffer sample to the undiluted sample. Do the same for your acid and base samples. Do you notice any difference between the buffer and the acid or base? Suggest a reason for any difference you see. A common biological buffer is a "phosphate buffer," which involves the equilibrium between H_ 2P0_ 4^- and HP0_ 4^-, with a P Ka at 25 degree C of 6.86. Describe in words how you would prepare 500. mL of 0.250 M phosphate buffer at pH = 7.2, given appropriate glassware and 500. mL each of 250 M Na H_ 2P0_ 4 and 0.250 M Na_ 2H P0_ 4 at 25 degree C. (Note: your explanation needs to include actual volumes, so this question requires calculations! It will be helpful to know that the concentrations of each component must add to the buffer concentration, i.e. [A^-] + [HA] = 0.250 M).Explanation / Answer
1. a. Handerson-Hasselbalch equation,
log [conj. base]/[conj. acid] vs. pH ---------> straight line
[conj. base]/[conj. acid] vs. pH ---------> S -shape curve
b. When a strong acid is added to a buffer, the acid reacts with the conjugate base and is completely consumed. Despite the addition of a strong acid, the pH of the buffer solution decreases only slightly.
Example: When H3O+ is added to a CH3CO2H/NaCH3CO2 buffer, it consumes some of the conjugate base, forming additional acetic acid.
H3O+(aq) + CH3CO2(aq) ----------> H2O() + CH3CO2H(aq)
Adding a strong acid such as HCl to water results in :
HCl + H2O H3O+ + Cl-.
The proton (H+) from the acid binds to neutral water molecules to form H3O+ raising the concentration of H+. The resulting large concentration of (H+) makes the solution more acidic and leads to a dramatic drop in the pH.
c. Dilution means, addition of water to acid.
If we dilute an acidic solution, the solution becomes less acidic. Its pH increases and approaches 7.
d. The more diluted the solution is, the more solution pH is dominated not by the presence of acetic acid and its conjugate base, but by the water autodissociation.