Question

You are instructed to create 400. mL of a 0.40 M phosphate buffer with a pH of 6.4. 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) H3O+(aq) + H2PO4−(aq)	Ka1 = 6.9 ✕ 10−3
H2PO4−(aq) + H2O(l) H3O+(aq) + HPO42−(aq)	Ka2 = 6.2 ✕ 10−8
HPO42−(aq) + H2O(l) H3O+(aq) + PO43−(aq)	Ka3 = 4.8 ✕ 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?
  M

What is the molarity needed for the base component of the buffer?
  M

How many moles of acid are needed for the buffer?
  mol

How many moles of base are needed for the buffer?
  mol

How many grams of acid are needed for the buffer?
  g

How many grams of base are needed for the buffer?
  g

Answer 1

The Ka values for the phosphoric acid are The phosphoric acid has the following ionizations H3PO4 = H2PO4 + H+ pka, = -log(6.9 x 10-3) = 2.16 H2PO4 = HPO4-2 + H+ pka = -log(6.2 x 10-8) = 7.21 HPO4-27 POA-3 + H+ pka, = -log(4.8 x 10-13) = 12.32 Since the given pH (6.4) is nearer to second ionization. The phosphate buffer must be prepared by H2PO4- 1 and HPO 4 - 2 Acid component: NaH2PO4 Base component: Na2HPO4 [base] pH = pK, +log [acid] [HPO,?] 36.4=7.21+10g TH,PO, 1 logh [HPO,?) log TH,PO, ] =-0.81 [HPO, ?) – 10–0.81 = 1.5574 [H,PO, 1 [HPO, ?]=0.15574 x [H,PO:-] Let the conc. of buffer = 0.40M That is [HPO, ?]+[H,PO, ]=0.40 0.15574x[H,PO _]+[H,PO-]=0.40 1.15574x[H,PO, ]=0.40 [H,PO,"]= 1,594 = 0.3461 [HPO, 2]=0.15574x[H,PO, ]=0.15574x0.3461 = 0.0539 [H,PO,-]=0.3461M [HPO, ?]=0.0539M Volume of the buffer solution = 400 mL = 0.4L Number of moles of H,PO, present in the solution = M*V = 0.3461M 0.4L = 0.138 mol Number of moles of HPO, present in the solution = M*V =0.0539M ~0.4L = 0.0216 mol Molecular weight NaH,PO4 = 119.98 g/mol Mass of NaH,PO, = molesx mol.wt. = 0.138 molx119.98 g/mol =16.6 g Mass of NaH,PO4 = 16.6 g Molecular weight of dibasic potassium phosphate (Na2HPO )=141.96 g/mol Mass of Na, HPO, = moles x mol.wt. = 0.0216 mol x 141.96 g/mol = 3.06 g Mass of Na HPO4 = 3.06 g molarity of acid component of the buffer = 0.346M molarity of base component of the buffer = 0.0539M moles of acid need for buffer = 0.138 mol moles of base need for buffer = 0.0216 mol mass of acid need for buffer = 16.6 g mass of base need for buffer = 3.06 g