Question

You are instructed to create 400. mL of a 0.39 M phosphate buffer with a pH of 6.2. You have phosphoric acid and the sodium salts NaH2PO4, Na2HPO4, and Na3PO4 available. (Enter all numerical answers to three significant figures.) H3PO4(s) + H2O(l) = H30+ (aq) + H2P04 (aq) Kai = 6.9 x 10-3 H2PO4 (aq) + H20(1) =H30+ (aq) + HPO42-(aq) Ka2 = 6.2 x 10-8 HPO42-(aq) + H20(I) = H30+(aq) + PO43-(aq) Ka3 = 4.8 x 10-13 Which of the available chemicals will you use for the acid component of your buffer? H3PO4 NaH2PO4 Na2HPO4 Na3PO4 Which of the available chemicals will you use for the base component of your buffer? H3PO4 NaH2PO4 Na2HPO4 Na3PO4 What is the molarity needed for the acid component of the buffer? What is the molarity needed for the base component of the buffer? How many moles of acid are needed for the buffer? mol How many moles of base are needed for the buffer? 49 mol How many grams of acid are needed for the buffer?

Answer 1

acid = NaH2PO4

Base = Na2HPO4

pH of phosphate buffer = pka2 + log(Na2HPO4/NaH2PO4)

   pka2 phosphate buffer = 7.21

pH = 7.4

total no of mol of buffer = M*V

               = 0.39*400/1000 = 0.156 mol

no of mol of NaH2PO4 required =   0.156-x mmol

no of mol of Na2HPO4 required =   x mol

6.2 = 7.21 + log(x/(0.156-x))

x = 0.014 mol

acid = NaH2PO4

no of mol of NaH2PO4 required =   0.156-x = 0.156-0.014 = 0.142 mol

Concentration of NaH2PO4 required = 0.142/0.4 = 0.355 M

mass of acid(NaH2PO4) needed = 0.142*120 = 17.04 g

base = Na2HPO4

no of mol of Na2HPO4 required = x = 0.014 mol

Concentration of NaH2PO4 required = 0.014/0.4 = 0.035 M