basic chemistry buffer problem. please help. This is a classic: the phosphate bu

Question

basic chemistry buffer problem. please help.

This is a classic: the phosphate buffer family. It is probably, along with citrate and carbonate systems, the most natural way to buffer the pH of solutions used in biological research. We have 12 discussed in recent lectures how molalities are used as reference frame to define chemical reaction equilibrium constants in solution. For reasonably low concentrations of solutes in aqueous solutions, concentrations in mol/L rather than molality (mol/kg water) are often adopted in the equilibrium constants. The titration curve on the right tells pretty much the whole story. The 1, definition of plc is, pKa = -log10Ka. Treat the system as an ideal 6 one i.e., you do not need to worry about activity coefficients, just concentrations. You are asked to calculate the concentrations of all four phosphorus-containing species in solution if, a) you dissolve 0.5 mol of H3PO4 and 0.5 mol Na3PO4 in water to a final volume of 1 L (4 points), and b) you dissolve 0.5 mol of NaH2PO4 and 0.5 mol Na2HPO4 in water to a final volume of 1 L (4 points). c) What's the pH (-log10[H+]) in each case? (2 points) To remain in tune with the Paper Reduction Act of 1995, I'm asking that you use this same sheet, front and back, for your answers as much as you can.

Explanation / Answer

1) If we dissolve H3PO4 and Na3PO4 in a soltion then

H3PO4 + PO4^3- => HPO4^2- + H2PO4^-
thus
buffer system of HPO4^2- + H2PO4^-

so we will use Ka2 for calculations, concentrations of both will be same

pH = pKa2 - log(0.5/0.5) = - lg(6,3*10^-8) = 7.2

b) pH = 7.2 + lgo [0.5 / 0.5] = 7.2

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