Lab: THE DETERMINATION OF CHLORIDE BY VOLHARD\'S METHOD 3. Which dissolved ions

Question

Lab: THE DETERMINATION OF CHLORIDE BY VOLHARD'S METHOD

3. Which dissolved ions are present in significant amounts in the titrated solution:
(a) during titration of the excess silver?
(b) after adding an excess of KSCN?

5. What would be the consequence of adding significantly more than 2 mL of the ferric ammonium
sulfate indicator?

6. Determine the %(w/w) of Cl in a solid sample. The sample (0.3721g) was treated with 50.00
mL of 0.02051M AgNO3 after dissolving it in H2O. The excess Ag+ was titrated with 24.21 mL
of 0.01997 M KSCN.

3. Which dissolved ions are present in significant amounts in the titrated solution: (a) during titration of the excess silver? (b) after adding an excess of KSCN?

Explanation / Answer

3. Dissolved ion present in significant amounts in the titrated solution:

(a) during titration of excess silver : Ag+ and NO3-

(b) after adding excess KSCN : K+, SCN- and NO3-

5. If excess ferric ammonium sulfate indicator is added, we would get the end point much before the actual end point and this would give error in the titration results.

6. moles of AgNO3 added = 0.02051 M x 50 ml = 1.0255 mmol

moles of KSCN reacted with excess Ag+ = 0.01997 M x 24.21 ml = 0.4835 mmol

moles of Ag+ reacted with Cl- = moles of Cl- = 1.0255 - 0.4835 = 0.5420 mmol

mass of Cl- present in sample = 0.5420 mmol x 35.45/1000 = 0.0192 g

%(w/w) of Cl in the solid = 0.0192 x 100/0.3721 = 5.16%

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