Question

Which of the structures below best represents an amino acid at neutral pH (i.e.,pH = 7)?

a. ⁺H₃NCHRCOOH

b. ⁺H₃NCHRCOO^-

c. H₂NCHRCOOH

d. H₂NCHRCOO^-

B. MOPS has a pKa of 7.2. To prepare 1.00 L of a 25.0 mM MOPS solution with a pH of 7.4, I first dissolved 25 mmol of MOPS base in 900 mL of deionized water. After adjusting the pH to 7.4 with 1.00 M HCl, I will dilute the solution to 1.00 L with additional deionized water. How much 1.00 M HCl wil I need to add to reach pH 7.4?

C. The Ksp of calcium phosphate [Ca3(PO4)2] is 1.2 x 10-26. If the free phosphate concentration of our intracellular fluid is, at equilibrium, no more than 3.5 μM (μ = 10-6). What is the highest concentration of calcium that will not cause precipitation?

Answer 1

A) The structure of amino acids at pH 7 depend on their side-chains and on pKa values. It can be any of the 4 given structures depending on what amino acid we are considering.

B) We can calculate the amount of HCl needed using Henderson-Hasselbalch equation, which is given as -

where n(HCl) is the number of moles of HCl.

Now, since molarity is define as ratio of moles to volume, we can calculate volume as -

C) Calcium phosphate ionizes as -

and the solubility product, Ksp is given by -

Since Ksp and equilibrium concentration of phosphate are given, we can calculate the concentration of calcium.