1 "HEPES" is an abbreviation for N-2-hydroxyethylpiperazine-N?-2-ethanesulfonic acid, a commonly used buffer in biochemistry. The structure and pKa of HEPES are given in Table 8-2 of Harris. Note that the form shown in the table is actually a zwitterion, with a protonated 3° amine (ammonium) and a deprotonated sulfonic acid (sulfonate). But since sulfonic acids have very low pKa values (they are essentially "strong"), we will ignore the sulfonic acid proton and treat the zwitterion as a monoprotic acid, "HA."
a.Write the net ionic equation that occurs when some strong acid is added to a HEPES buffer. Include physical states.
b. Write the net ionic equation that occurs when some strong base is added to a HEPES buffer. Include physical states.
c. Calculate the fractional compositions ? 0 (HA) and ? 1 (A?) for HEPES at pH = 7.00, 7.56, and 7.80. Assume the ionic strength is zero. [Note: monoprotic fractional composition equations are explained on p. 28 of the Equation Guide.]
d. Consider a 0.100 M (total concentration) HEPES buffer at pH 7.80. Calculate the concentrations of HA and A?, then mmol of HA and A? in 50.0 mL of this buffer.
e. Calculate the pH that results when 1.00 mL of 1.00 M HCl is added to 50.0 mL of the buffer in part d. [Hint: how many mmol of H+ was added, and what does it this added H+ do?]
f. Calculate the pH that results when 1.00 mL of 1.00 M NaOH is added to 50.0 mL of the buffer in part d.