GRAVIMETRIC ESTIMATION OF CHLORIDE IONS

1. Aneeqa Haider, Ariel Tsang, Carrie Fan, Fabiha Nuzhat GRAVIMETRIC ESTIMATION OF CHLORIDE IONS

2. Introduction • Chloride ion • Cl + e−  Cl− • Chloride • Results from the combination of Cl2 with a metal (e.g. NaCl) • Reactants: NaCl, AgNO3 Chloride Ion

3. Introduction • Gravimetric analysis • The set of procedures to determine the quantity of a substance present in the mass of a solid • Seven steps • Drying and measuring the masses of samples to be analysed • Dissolving the sample in distilled water. • Precipitating the substance by adding a reagent. • Separating the precipitate from the solution by filtration. • Washing precipitate free of impurities. • Drying precipitate to obtain mass. • Determine the amount of the original ion based on the known mass and composition of the precipitate.

4. Introduction • Objective: Determine the amount of chloride ions present in a given solution of NaCl using AgNO3 as a reagent. • Problem: What is the amount of chloride ions present in 0.2 g of Sodium Chloride (NaCl)? • Hypothesis: The number of chloride ions present in 0.2 g of NaCl is approximately 2.06 x 10²¹.

5. Materials • Erlenmeyer Flask (2) • Beaker (1) • Funnel (1) • Ashless Filter Paper (1) • Paper Clips (4) • Balance • 0.2 g of Sodium Chloride -NaCl • 3 g of Silver Nitrate - AgNO3(aq) • Distilled Water • Dropper • Test Tubes (2) • Bunsen Burner • Crucible and lid • Crucible tongs • Retort Stand • Ring Clamp • Clay Triangle • Safety Goggles • Spatula • Stirring Rod • Graduated Cylinder

6. Procedure • Formation of the precipitate • Filtration of the solution containing the precipitate • Measurement of the mass of AgCl by drying the filter paper • Measurement of the mass of AgCl by burning the ashless filter paper

7. Formation of the Precipitate • 0.2 g of NaCl was dissolved in Distilled Water in Erlenmeyer Flask • 3.4 g of AgNO3(aq) was poured into Erlenmeyer Flask containing NaCl (aq). • Solution was put into rest until all the precipitate formed.

8. Filtration of the Solution containing the Precipitate • Filter paper, funnel and Erlenmeyer flask were set up • The solution containing the precipitate was poured through the filter paper • Washed periodically with Distilled water

10. Measurement of the Mass of AgCl by drying the filter paper • Filter paper and precipitate were completely dried • Mass of the precipitate: • Mass of the filter paper with precipitate - Mass of the filter paper

11. Measurement of the Mass of AgCl by burning the ashless filter paper • Retort stand, ring clamp, clay triangle, and Bunsen burner were set up • Filter paper was carefully folded with the precipitate inside, and placed in the crucible • Crucible was heated until no more filter paper was left • Mass of Precipitate: • Mass of crucible, lid & precipitate – Mass of crucible & lid

13. Safety Precautions • General Safety Precautions • Safety precautions specific for this experiment: • Avoiding contact with Silver Chloride (AgCl) • Safety precautions while using the Bunsen burner

14. Observations • Formation of the Precipitate

15. Observations • Measurement of the mass of AgCl by drying the filter paper

16. Observations • Measurement of the mass of AgCl by drying the filter paper

17. Calculations – Known Information • Mass used of Sodium Chloride (NaCl): • 0.2 g • Molar mass of NaCl: • 35.45 g/mol • Percentage composition by mass of Silver Chloride (AgCl): • Silver (Ag) = 75% • Chloride (Cl) = 25%

18. Calculations – By Drying Filter Paper • Mass of filter paper: • 1.04g • Mass of filter paper + AgCl: • 1.43g • AgCl: • (1.43g – 1.04g) = 0.39g • Mass of chloride ions present: • 0.25 x 0.39g = 0.0975g

19. Calculations – By Drying Filter Paper • Number of moles of chloride ions: = Mass of Cl Molar Mass of Cl = 0.0975g 35.45g = 0.00275 mol

20. Calculations – By Drying Filter Paper • Mole = Avogadro's number • 6.022 x 1023 • Number of chloride ions: = (# of moles) x (Avogadro's number) = (0.00275) x (6.022 x 1023) = 1.656 x 1021chloride ions present in 0.2 g of NaCl by drying filter paper

21. Calculations – By Burning Ashless Paper • Mass of crucible + lid + filter paper: • 32.13g • Mass of crucible + lid + filter paper + AgCl • 32.64g • AgCl • (32.64g – 32.13g) = 0.51g • Mass of chloride ions present: • 0.25 x 0.51g = 0.1275g

22. Calculations – By Burning Ashless Paper • Number of moles of chloride ions: = Mass of Cl Molar Mass of Cl = 0.1275g 35.45g =0.00360 mol

23. Calculations – By Burning Ashless Paper • Mole = Avogadro's number: • 6.022 x 1023 • Number of chloride ions: • (# of moles) x (Avogadro's number) • (0.00360) x (6.022 x 1023) • 2.167 x 1021chloride ions present in 0.2 g of NaCl by burning filter paper

24. How did the Law of Conservation of Mass help predict the amount of Cl ions in AgCl? • Law of Conservation of Mass • Mass of the reactants = Mass of the products • Mass of Cl ions in NaCl (reactant) = Mass of Cl ion is AgCl (product)

25. What type of chemical reaction is taking place in this experiment? • Double displacement reaction: AB + CD → AD + CB  AgNO3 (aq) + NaCl (aq) → AgCl (s) + NaNO3 (aq)

26. Why does NaCl dissolve in water? • Hydration provides greater stability than lattice energy • Hydration shell Water – dipole moment

27. Why doesn’t AgCl dissolve in water? • More stable as a solid precipitate than separate ions • Hydration energy provided is less than lattice energy released when compound forms

28. What are some properties of AgCl? • White crystalline solid • Light sensitive • Purple  black • Change colour when AgCl  Ag + Cl • Dry powder, doesn’t draw moisture from the air • Very low solubility • MP: 455°C BP: 1550°C

29. Why doesn’t an excess amount of AgNO3 affect the chemical reaction in the experiment? • AgNO3 is the excess reagent • Amount of chloride ions = product • In this double displacement reaction all of the NaCl must be used up • LR = NaCl : limits amount of AgNO3 used limits amount of products • An excess of AgNO3 will not react since all the NaCl is used up already

30. Why was it necessary to wash off all the impurities from the AgCl precipitate? • Otherwise, mass of the impurities would be included in the mass of the AgCl precipitate • Causes an inaccurate measurement of mass of AgCl • Causes inaccurate determination of the number of Cl ions

31. Why were the contents of the crucible slightly gray in colour after heating? • Contents: AgCl and ashless filter paper • Ashless filter paper turned into CO2 • Remaining content: AgCl • AgCl is a white coloured powder at SATP • Upon heating, AgCl undergoes decomposition to yield Ag and Cl

32. Percentage Yield and Error • Percentage Yield = Actual Yield x 100 Theoretical Yield = 80% • Percentage Error = (Theoretical Yield – Actual Yield) x 100 Theoretical Yield = 20%

33. Conclusion • By drying filter paper: • Number of chloride ions in 0.2g of NaCl is 1.656 x 1021 • By burning ashless filter paper: • Number of chloride ions in 0.2g of NaCl is 2.167 x 1021 • Amount of ions present in NaCl = amount of ions present in AgCl • Law of Conservation of Mass

34. Sources of Error • The reading on the electrical balance was observed to change constantly • Due to slight air currents • Contents being weighted were extremely light • Measured multiple times • Small amounts of AgCl were stuck in the flask after attempts to remove it • Caused alterations in final mass • Inaccurate percentage yield

35. Sources of Error • Ashless filter paper not burned away completely • final mass greater than expected • Crucible was placed on counter to allow for cooling • picked up unwanted particles on the counter

36. Suggestions • Use of better quality ashless filter paper that will completely burn away without leaving any unwanted residue • Use of clean crucible and lid • Minimum transfer of the samples from container to container

37. THE END.Thank you for listening to our presentation.