I want to make 1.0 liter of a buffer that will keep the pH of a solution at 7.40

Question

I want to make 1.0 liter of a buffer that will keep the pH of a solution at 7.40. If I start by dissolving 30 g of sodium dihydrogen phosphate in water, how many milliters of either 0.75 M HCI, or 0.75M NaOH will I have to add to this solution to make my buffer? For phosphoric acid: K_a1 = 7.11 times 10^-3; Ka_2 = 6.32 times 10^8; Ka_3 = 4.5 times 10^-13 Calculate the theoretical potential of the cell Zn/ZnSO_4 (0.05M), PbSO_4 (saturated)/Pb (a) using concentration; (b)using activity. PbSO_4 (s) + 2e^- rightarrow Pb(s) + SO_4^2- E^0 = -0.350V Zn^2+ + 2e^- rightarrow Zn(s) E^0 = -0.763 V A. 2.16 g sample containing both Fe and V was dissolved under certain conditions and diluted to 500.00 mL. A first 50.00 mL aliquot was taken and passed through a Walden reductor to form Fe^2+ and VO^2+ ions. The titration of this solution required 16.84 mL of 0.1000 M Ce^4+ to reach end point. A second 50.00 mL aliquot was passed through a Jones reductor to form Fe^2+ and V^2+ ions. The titration of the second solution required 44.26 mL of 0.1000 M Ce^4+ solution to reach an end point. Calculate the percentage of Fe and V in the sample. Derive a titration curve for 30.00 mL of 0.0500 M Co^2+ with buffered to pH 10. Determine the pCo values after the addition of 10.00, 15.00 and 20.00 mL of EDTA. K_MY (CoEDTA) = 2.82 times 10^16.

Explanation / Answer

1. pH of buffer = 7.4

pKa2 = 7.2

initial NaH2PO4 = 30/119.98 = 0.25 mol

let x moles of NaOH has been added

using Hendersen-Haselbalck equation,

pH = pKa + log(base/acid)

      = pKa2 + log(HPO4-/H2PO4-)

7.4 = 7.2 + log(x/0.25-x)

x = 0.4/2.6 = 0.154 mol

Volume of NaOH to be added = 0.154 x 1000/0.75 = 205.33 ml

2. Using Nernst equation

Ecell = Eocell = -0.0592/n logK

         = (-0.350 - (-0.763)) - 0.0592/2 log(0.05)

         = 0.4515 V

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