Concentration of blood-enzyme with histidine side chain

Question

1. A solution of 0.100 M acetic acid is prepared.

a) What is its pH value?

b) If 20% of the initial acetic acid is converted to the acetate form by titration with NaOH, what is the resultant pH?

c) At what pH value would one expect to find a 1:1 ratio of the acetate and acetic acid forms?

d) At what pH value would one anticipate to find 10% of the total species (acetate + acetic acid) in the basic form?

e) At what pH value would one anticipate to find 90% of the total acetic acid species in the basic form?

2. Please refer to Section 1.1 – A Closer Look on page 9 of the text. Also consult A Closer Look 1.2 on page 10.

a) If initially the blood pH is 7.40, what is the concentration of CO₂ in blood, assuming the pK’ and bicarbonate level in the section entitled A Closer Look 1.2?

b) What would be the consequence on blood pH of a 10% increase in the concentration of blood CO₂?

c) If one wanted to return to initial pH of 7.40, given the change in concentration of CO₂, to what concentration would the blood HCO₃- need to be adjusted?

d) A patient has a blood concentration of CO₂ of 0.6 mM. If the blood pH is 7.40, what is concentration of bicarbonate ion in the blood? If the blood [CO₂] rises by 20% due to physiology, what is the new pH of the blood? What [HCO₃-] would be required to keep the blood pH of 7.40, if the physiology keeps the [CO₂] elevated?

3. Your technician comes to you to ask for help. You’ve asked that a pH 7.20 buffer of Na₂HPO₄/NaH₂PO₄ of total concentration 0.2 M be prepared. He tells you that he has found a stock solution of 0.2 M NaH₂PO₄ and asks how to make the buffer. He also informs you that he has no Na2HPO4 on hand, but that there is a supply of both concentrated HCl(aq) and NaOH(aq) in the lab. The lab is equipped with a pH meter.

Supposing that any amount of strong acid or base that can be added will not dilute the stock solution by much, what guidance can you give him? All that you want to achieve is the pH 7.20 (as measured by the pH meter) buffer at a total concentration of about 0.2 M. Is there a “quick and dirty” way to obtain the desired buffer? What will be the ratio of the two relevant phosphate forms if you have achieved the intended result? Figure 1.6 and the information on page 10 of the text should be of great help to you.

4. You are studying an enzyme with a histidine side chain that is included in enzymatic catalysis. It is thought that the basic form is important in catalysis. So, as part of your investigation of this enzyme, you wish to estimate the ratios of the basic and acidic forms of the histidine side chain at pH values of 6.0, 7.50 and 8.00. Assuming that the pK of the histidine in the enzyme protein is approximately equal to that in the free amino acid, calculate numerical estimates of the ratios of the basic to the acid forms of the histidine residue side chain at the given pH va