Question

SALTS: For each of the following salt solutions, record the experimental pH, determine the theoretical pH (show your work) and then determine the percent error between the two (show your work). For errors of greater than 20%, provide an explanation for the deviation.

1) 0.15 M KCl Solution; experimental pH = 6.82

Acid	Ka
HCL	>>1

Calculated pH: _____________ Actual pH: ______________ Percent error: ____________

2) 0.15 M Na2HPO4 Solution; experimental pH = 8.90

Acid	Ka
H3PO4	7.1 x 10-3
H2PO4-	6.3 x 10-8
HPO42-	4.5 x 10-13

Calculated pH: _____________ Actual pH: _______________ Percent error: _____________

3) 0.15 M NH₄NO₃; experimental pH = 6.90

Acid	Ka
HNO3	>>1
NH4+	5.6 x 10-10

Calculated pH: _____________ Actual pH: ______________ Percent error:____________

Please show calculations! I am very confused.

Answer 1

1) For KCl

salt of strong acid and strong base would have a theoretical pH = 7

Percent error = (7 - 6.82) x 100/7 = 2.60%

2) 0.15 M Na2HPO4

HPO4- + H2O <==> H2PO4- + OH-

1 x 10^-14/6.16 x 10^-8 = x^2/0.15

x = [OH-] = 1.56 x 10^-4 M

pOH = -log[OH-] = 3.80

pH = 14 - pOH = 10.19

Percent error = (10.19 - 8.90) x 100/8.90 = 14.50%

3) 0.15 M NH4NO3

NH4NO3 --> NH4+ + NO3-

NH4+ + H2O --> NH3 + H3O+

1 x 10^-14/1.8 x 10^-5 = x^2/0.15

x = [H3O+] = 9.13 x 10^-6 M

pH = -log[H3O+] = 5.04

Percent error = (6.9 - 5.04) x 100/6.9 = 26.96%