Distilled water frequently contains dissolved Co2, making the pH different from
ID: 772477 • Letter: D
Question
Distilled water frequently contains dissolved Co2, making the pH different from the the oretical value for pure water. Would this cause the pH of your buffer to be slightly higher or slightly lower than the target pH ? Explain. Was the pH of the buffer you prepared slightly higher or slightly lower than the target value ? the pH of a buffer can be adjusted by added small amounts of HCI or NaOH. Which of the se should be added to your buffer to remedy the problem identified in (a) ? Defend your choice. Consider the HCIO / CIO" conjugate acid / base pair. Consult your text or other source for appropriate acid dissociation constants. the hypochlorite ion is the active ingredient in bleach. NaCIO(aq). Explain why this acid / base pair would be a candidate for a buffer with pH = 7. 3 (approximate physiological value). Based on your advice in (a), a researcher used this very buffer in a growth medium (including appropriate food and temperature control) for bacteria that proliferate at physiological pH. Despite this carefully prepared medium, however, bacterial growth was dismal. Explain. Biochemists frequently use "phosphate" buffers in the ir work. Here, "phosphate" refers generally to the class of phosphate derived ions: H2PO 4 - , HPO 4 2 - , PO 4 3 - . Which of the se ions best represents the conjugate acid in a "phosphate" buffer at physiological pH ? Explain. A buffer used in an experiment was described as "a 0. 2 M phosphate buffer with pH = 7. 25". Based on the conjugate acid your identified in (a), determine the concentration of conjugate acid in this "0. 2 M phosphate buffer".Explanation / Answer
1).Dissolved carbon dioxide is correlated with the pH scale. The carbon dioxide dissolved in water has even more effect than the oxygen. Oxygen remains as an O2 molecule, whether it's in its gas phase or in solution, but when CO2 is dissolved in water, a small proportion of it reacts chemically with H2O to form carbonic acid, H2CO3. In water carbonic acid dissociates rapidly to form a H+ ion and HCO2 (bicarbonate), so it affects the carbonate equilibrium, and pH values change as a result.
2a) pH of the buffer was lower than the target.
b) NaOH should be added to increase the pH slightly.
3) acid dissociation constant Ka= 3 x 10-8.
b) pKa = - log Ka = 7.46
pH = pKa + log [ClO-] / [HClO]
7.3 = 7.46 + log [ClO-] / [HClO]
-0.16 = log [ClO-] / [HClO]
10^-0.16 = [ClO-] / [HClO] =0.575
Since weak acids and bases are used to make buffers,the ratio of concentration of acid and base is almost half so pH will be maintained.
b) The pH might got change in the presence of CO2 in aqueous solution of NaClO.
4).a) H2PO4- is a conjugate acid of HPO4(2-) after addition of H+.
HPO4(2-) + H+ ----> H2PO4-
b) pH=pKa+ log(buffer/conjugate acid)
pH = pKa + log ([HPO42-]/[H2PO4-])
7.25 = -log (6.31 x 10^-8) + log ([HPO42-]/[H2PO4-])
7.25 = 7.2 + log ([HPO42-]/[H2PO4-])
0.05 = log ([HPO42-]/[H2PO4-])
10^0.05 = ([HPO42-]/[H2PO4-]) = 1.12
[H2PO4-]=2/1.122= 1.78M