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CHM 112
Section 1

Homework Assignment #17
Due March 22, 2006 by 8:05 a.m.

Practice (not to be turned in): Problems 15.63, 15.64, 15.65, 15.67

Determine if the pH < 7 or pH > 7 (at 25 oC) for aqueous solutions of the following salts:

0) MnBr2

1) Cr(NO3)3

2) NiI2

3) Cu(ClO4)2

4) CoCl2

5) NaHSO4

6) NaH2PO4

7) Na2HPO4

8) Na3PO4

9) Na2HAsO4

Show all the relevant chemical reactions that take place in the solution.



Answers:

0) MnBr2

MnBr2(aq) Mn2+(aq) + 2 Br(aq)

Mn2+(aq) + 2 H2O(l)MnOH+(aq) + H3O+(aq)

Br(aq) + H2O(l)NR

pH < 7



1) Cr(NO3)3

Cr(NO3)3(aq) Cr3+(aq) + 3 NO3(aq)

Cr3+(aq) + 2 H2O(l)CrOH2+(aq) + H3O+(aq)

NO3(aq) + H2O(l) NR

pH < 7



2) NiI2

NiI2(aq) Ni2+(aq) + 2 I(aq)

Ni2+(aq) + 2 H2O(l)NiOH+(aq) + H3O+(aq)

I(aq) + H2O(l) NR

pH < 7



3) Cu(ClO4)2

Cu(ClO4)2(aq) Cu2+(aq) + 2 ClO4(aq)

Cu2+(aq) + 2 H2O(l)CuOH+(aq) + H3O+(aq)

ClO4(aq) + H2O(l)NR

pH < 7



4) CoCl2

CoCl2(aq) Co2+(aq) + 2 Cl(aq)

Co2+(aq) + 2 H2O(l)CoOH+(aq) + H3O+(aq)

Cl(aq) + H2O(l)NR

pH < 7



5) NaHSO4

NaHSO4(aq) Na+(aq) + HSO4(aq)

Na+(aq) + 2 H2O(l)NR

HSO4(aq) + H2O(l) H3O+(aq) + SO42–(aq)

pH < 7



6) NaH2PO4

NaH2PO4(aq) Na+(aq) + H2PO4(aq)

Na+(aq) + 2 H2O(l)NR

H2PO4(aq) + H2O(l) H3PO4(aq) + OH(aq)    Kb = 1.0×10–14/7.1×10–3 = 1.4×10–12

OR

H2PO4(aq) + H2O(l)H3O+(aq) + HPO42–(aq)    Ka = 6.3×10–8

pH < 7 since Kb < Ka



7) Na2HPO4

Na2HPO4(aq) 2 Na+(aq) + HPO42–(aq)

Na+(aq) + 2 H2O(l)NR

HPO42–(aq) + H2O(l) H2PO4(aq) + OH(aq)    Kb = 1.0×10–14/6.3×10–8 = 1.6×10–7

OR

HPO42–(aq) + H2O(l) PO43–(aq) + H3O+(aq)    Ka = 4.2×10–13

pH > 7 since Kb > Ka



8) Na3PO4

Na3PO4 3 Na+(aq) + PO43–(aq)

Na+(aq) + 2 H2O(l)NR

PO43–(aq) + H2O(l) HPO42–(aq) + OH(aq)

pH > 7



9) Na2HAsO4

Na2HAsO4(aq) 2 Na+(aq) + HAsO42–(aq)

Na+(aq) + 2 H2O(l)NR

HAsO42–(aq) + H2O(l) H2AsO4(aq) + OH(aq)    Kb = 1.0×10–14/1.0×10–7 = 1.0×10–7

OR

HAsO42–(aq) + H2O(l) AsO43–(aq) + H3O+(aq)    Ka = 3.2×10–12

pH > 7 since Kb > Ka