You have been given a sample of seawater, a pipet that contains 5.0 × 10–4 mole/gram AgNO3, a pipet — Precipitation Titrimetry Chemistry Question

At least two trials were attempted using both of the filled AgNO₃ pipets and averaging results. These titrations were completed in the small test tubes provided. A clear and organized table showed the initial and final masses of both the seawater and silver nitrate pipets. Use of the Ksp values was made to clearly show the AgCl then Ag₂CrO₄ selective order of precipitation.

Sample Data:
Initial mass of the seawater and pipet: 3.121 g
Final mass of the seawater and pipet: 2.243 g
Initial mass of silver nitrate and pipet: 2.820 g
Final mass of silver nitrate and pipet: 1.918 g

Sample Student Calculations:
3.121 g – 2.243 g = 0.878 g seawater used
2.820 g – 1.918 g = 0.902 g silver nitration solution used
Molar masses: NaCl = 58.44 g/mol; AgNO₃ = 169.87 g/mol
0.902 g AgNO₃ x 5.0 x 10^-4 mol/g AgNO₃ = 4.51 x 10^-4 mole AgNO₃
4.51 x 10^-4 mole AgNO₃ = mole Ag⁺ ; mole Cl^-
4.51 x 10^-4 mole Cl^- x 35.5 g/mol Cl^- = 1.60 x 10^-2 g Cl^-
1.60 x 10^-2 g Cl^- / 0.878 g seawater x 100 = 1.82% Cl^- in solution by mass

1.82% Cl^- in solution by mass (A value of 1.80-1.90% is generally accepted)

Assumptions included the complete precipitation of AgCl from solution, the equivalent molar ratio of Ag⁺: Cl^-, the presence of a drop or two of the chromate solution to the seawater to be sufficient to create a white/red color change endpoint. Error included the subjective judgment of the endpoint to distinguish the complete precipitation of the chloride from the beginning of the precipitation of the chromate, and the additional drop or two overshooting of the endpoint to turn the solution entirely reddish.