At neutral pH Valine can be thought of as a n amphoteric species because it can

Question

At neutral pH Valine can be thought of as a n amphoteric species because it can donate the proton from its -COOH group and it can accept a proton on its -NH_2 group. At low pH Valine can act as a diprotic acid (which means that it can donate 2 consecutive protons) and at high pH it can act as a diprotic base (which means it can accept 2 consecutive protons). The titration curve to the right corresponds to the titration of diprotic Valine with 0.1 M NaOH. Use the titration curve to estimate the two pK_ a values of valine. a. 1.4 and 3.4 b. 2.4 and 9.7 c. 2.4 and 6.0 d. 6.0 and 11 e. 9.7 and 11 Which of the following represents valine in its most addle form (as a diprotic acid)? If you have a solution of valine at pH 9.0, approximately how many moles of 0.1 M NaOH can you add before you exceed the buffer capacity? a. 0.0001 moles b. 0.0003 moles c. 0.0005 moles d. 0.0007 moles e. 0.0009 moles At pH 1.8 Valine exists in its most acidic, diprotic form. How many mL of 0.2 M NaOH will you need to add to a solution of valine at pH 1.8 to fully deprotonate the Valine? a. 5 mL b. 10 mL c. 20 mL d. 40 mL e. 60 mL

Explanation / Answer

Q28

the titration is for an amino acid, which contains acidic and basic species

so

for the pKa, reclal that ALWAYS

Equivalent Volume = 10 mL

half equivalence = 10/2 = 5 mL

the pH in this point is pretty special since

pH = pKa + log(A-/HA)

and A = HA , so A-/HA = 1, given log(1) = 0

so pH = pKA

the pH at V = 5 mL ---> pH = 2 approx.. nearest answer is 2.4

for the second pK value...

this must be between pHequvalence 1 and pH equivalence 2

so

V1 = 10 mL and V2 = 20 mL

pH in V = 15 mL --> pH = 9.7 approx

so

best answer is B, 2.4 and 9.7

Q29

the most acidic form... will be the specie with most H+ ions present

so

ignoreb and d, since amino groups are not protonated

ignore c, since O- is not protonated as OH

so best answer is A, whic has OH and NH3+ group (most protonated)

For Q30 and Q31

please add the "exceeding" criteria for the buffer, since this could be either pH = +/- 1 or any ohter value

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