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Chapter 1: Amino Acids, Carbohydrates, Lipids, Nucleic Acids

Which of the following correctly defines the term pKa?

A. equilibrium constant for the dissociation of HA to A- and H+

B. ion constant of water

C. negative log of the concentration of H+

D. pH at which a molecule is neutrally charged

E. pH at which an equivalent distribution of acid and conjugate base exist in solution

Answer E: The logarithmic measure of the acid dissociation constant of an acid or base, termed pKa, is defined as the pH at which the protonated and unprotonated molecular species are at equal concentrations. With respect to this question the protonated species can be represented as HA while the unprotonated species would be A-.

Which of the following correctly defines the isoelectric point (pI) of an amino acid or protein?

A. the equilibrium constant for the ionization of the substance

B. the ion constant of water

C. negative log of the concentration of H+

D. pH at which a molecule is electrically neutral

E. pH at which an equivalent distribution of acid and conjugate base exists in solution

Answer D: The isoelectric point is that pH at which a substance exhibits no net charge. In other words, all the negative and positive charges, say for instance in a protein, are equal in number such that the molecule is electrically neutral.

The blood contains many compounds that serve to buffer the pH of the fluid such as bicarbonate and phosphate ions. Which of the following most correctly defines the meaning of the term buffering?

A. a solution containing a large concentration of a base such that the pH will not change significantly when an acid is added

B. a solution containing a large concentration of an acid such that the pH will not change significantly when more acid is added

C. a solution or substance which resists changes in pH when small quantities of an acid or base are added to it

D. pH at which a molecule or solution is neutrally charged

E. pH at which an equivalent distribution of acid and ...

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