You must show and submit all work for the following problems at the beginning of

Question

You must show and submit all work for the following problems at the beginning of your lab period during the week of 3/12/18. Work individually The instructor will not give full credit for any problem that is identical to another person's work or that does not show the logical analysis, even if the numerical result is correct. These problems are worth 20 points. Be sure to use proper units and significant figures. Remember tables of acid/base dissociation constants are located in AppendixD.1 & D.2 p. 1092-3 in your textbook. Report all pH's to +/-0.XX 3. You are given a 4.00 M solution of H PO and a 2.00 M solution of HPOHow many mL's of each solution should be placed into a 2.00 L volumetric flask in order to generate a 0.100 M buffer at pH = 6.95 if the pKa of H2PO4, is 7.21? Be sure to determine the concentrations of the buffer components and the necessary ml's of each solution.

Explanation / Answer

3. pH of buffer = 6.95

pKa = 7.21

using hendersen-hasselbalck equation.

pH = pKa + log(HPO4^2-/H2PO4-)

6.95 = 7.21 + log(HPO4^2-/H2PO4-)

(HPO4^2-) = 0.55(H2PO4-)

given,

(H2PO4-) + (HPO4^2-) = 0.1 M x 2.00 L = 0.2 moles

or, (H2PO4-) + 0.55(H2PO4-) = 0.2

(H2PO4-) = 0.2 moles/1.55 = 0.13 moles

Volume 4.00 M H2PO4- to be taken = 0.13 moles x 1000/4.00 M = 32.5 ml

(HPO4^2-) = 0.2 - 0.13 = 0.07 moles

Volume 2.00 M H2PO4- to be taken = 0.07moles x 1000/2.00 M = 35 ml

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