Suppose a solution contains a mixture of triprotic phosphoric acid, H_3PO_4, and

Question

Suppose a solution contains a mixture of triprotic phosphoric acid, H_3PO_4, and monoprotic hydrochloric acid, HCl, with the molarity of H_3PO_4 equal to 0.0784 M and the molarity of HCl equal to 0.0986 M. When a mixture of these two acids is titrated with a strong base, the first observable equivalence point occurs when both the HCl and the first H^+ from H_3PO_4 have been neutralized. A 2^nd equivalence point is observed when the 2nd H^+ from the phosphoric acid has been neutralized. If the molarity of the NaOH titrant used for this titration is 0.1034 M, calculate the expected volume of base (in mL) needed to reach the 1^st and 2^nd equivalence points when the base is used to titrate a 10.00 mL sample of this mixture. volume of base to reach 1^st equiv. pt. __________ (total) volume of base to reach 2^nd equiv. pt. _________

Explanation / Answer

total H+ initially = 2 (H+ form HCl and H+ from H3PO4)

so

mmol of NaOH = 10*0.0784+0.0986*10 = 1.77mmol of H+

1.77 mmol of NaOH

V = mmol/m = 1.77/(0.1034) = 17.12 mL of acid for FIRST point --> 1st point

for second point

mmol of NaOH = 1 H+ from H2PO4-

mmol of NaOH = M1*V1 = 10*0.0784 = 0.784 mmol of NaOH

V = mmol/mL = 0.784 / 0.1034 = 7.5822 mL --> 2nd poin

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