3D) (18 pts) You are given stock solutions of 1.00 M HCI, and 1.00 M NaOH, and a

Question

3D) (18 pts) You are given stock solutions of 1.00 M HCI, and 1.00 M NaOH, and a bottle of MOBS (a water soluble white powder, MW-223.29, structure as shown in the figure on the previous page), and told to prepare 2.00 L of a buffer that is 50 mM in MOBS species and has pH-7.6 for performing a restriction enzyme experiment. Perform a calculation of the quantities of MOBS, HCI and NaOH you would add to a solution before diluting to 2.00 L total volume to prepare this pH-7.6 buffer mass MOBS Vol HCI Vol NaOH

Explanation / Answer

pKa MOBS = 7.48

MOBS-/MOBS = base/acid

let x amount of 1.0 M NaOH is added to prepare buffer

x moles NaOH forms x moles MOBS- species in solution

Total MOBS = (MOBS) + (MOBS-) = 0.05 M x 2.0 L = 0.10 moles

Using Hendersen-Hasselbalck equation,

pH = pKa + log(MOBS-/MOBS)

7.6 = 7.48 + log(x/(0.10-x))

x = 0.076 - 0.76x

x = 0.076/1.76 = 0.0432 moles

Volume of 1.0 M NaOH required = 0.0432 moles x 1000/1.0 M = 43.2 ml

mass MOBS to be taken = 0.10 moles x 223.29 g/mol = 22.33 g

Vol HCl = none

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