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You may work all of the problems or one problem. Only one will be graded. Select the one problem you want graded. You must show work for credit. Our blood is buffered to maintain a pH of 7.4 +/- 0.05. Two buffer systems are found in the cells and blood plasma. HCO_3-/H_2CO_3 Ka = 4.22 times 10^-7 and HPO^-2_4:/H_2PO_4-Ka = 6.2 times 10^-8 Show the equations for the ionizations of these weak acids. Calculate the ratio of base/acid needed to achieve a pH of 7.4 with each buffer system. Which is the more effective buffer system for human blood? Why? A lab technician was preparing a buffer for use with an experimental synthetic blood system Unfortunately, the labels were mixed up and H_3PO_4 was mixed with H_2PO_4-. The Ka for this system is 7.1 times 10^-3. What will the pH of this system be if the ratio of [salt]/[acid] is 1.0. Can the ratio of the [salt]/[acid] in (d) be adjusted to give a pH of 7.4? Show why or why not.

Explanation / Answer

a)

H2C03 ---> H+ + HC03-

H2P04- --> H+ + HP04-2

b)

now

we know that

pH = pKa + log [ base /acid ]

also

pKa = -log Ka

so

pH = -log Ka + log [ base /acid ]

for HC03-/H2c03

7.4 = -log 4.22 x 10-7 + log [ base /acid ]

[ base /acid ] = 10.6

now

for HP042- / H2P04-

7.4 = -log 6.2 x 10-8 + log [ base /acid ]

[ base /acid ] = 1.557

c)

HP042- / H2P04- is more effective buffer

because

the ratio of [ base /acid ] is very small

also pKa is very close to pH of blood

d)

pH = -log Ka + log [ salt / acid]

pH = -log 7.1 x 10-3 + log 1

pH = 2.1487

so

pH of this system is 2.1487

e)

7.4 = 2.1487 + log [salt / acid ]

[salt /acid ] = 1.783 x 10^5

so

the ratio is very very large

so

it is not possible to adjust to given a pH of 7.4

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