25.0 of 0.0015M nitrous acid (HNO2) is titrated with 0.0012M dichromate ion (Cr2

Question

25.0 of 0.0015M nitrous acid (HNO2) is titrated with 0.0012M dichromate ion (Cr2O7 2-). The titration is monitored with a potentiometer using a SCE reference electrode ( E ref = 0.241V). The solution is buffered at pH=4.000. Determine the potential of the solution when 9.00ml of dichromate have been added.

Cr2O7 2-(aq)  +  14H+  6e-  -->  2Cr3+  + 7H2O    E= 1.36V

3HNO2  + 3H2O  -->  3NO- 3  +  9H+  +  6e-         E= 0.940V

Balance Net Eq: Cr2O7 2-(aq)  +  5H+  3HNO2  -->  2Cr3+  4H2O  + 3NO- 3

(3.75 x 10^-5 mol HNO2)(1mol dichromate/3molHNO2) = 1.25 x 10^-5 mol Cr2O7 2-

Explanation / Answer

E=E0-RT/nF(ln K)

=2.03-(.0831*300*2.303)/(2*95000)*ln (1/(1.25*10^-5)*3.75*10^-5)

=2.02 V

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