Question

5. When a sample of 0.242 grams of an unknown substance containing chloride (Cl-) was boiled and was analyzed and it was found to contain 0.084 gram of Cl- ion. Calculate the % chloride in in the substance.

6. Below is the overall equation of the reaction of Co 3+ with thiosulfate.

2 Co(NH3)4Cl3 + 2 S2O3 2- ----------> 2 Co 2+ + S4O6 2- + 8 NH3 + 6 Cl –

Use the overall equation to calculate what size sample should be measured so as to react with 25 mL

0.1500M Na2S2O3.

Answer 1

5. Mass of substance = 0.242 g

Mass of Cl- ions = 0.084 g

% Chloride in substance = Mass of Cl- / Mass of substance

= 0.084 / 0.242 * 100

= 34.7 %

6. The overall equation is:

2 Co(NH₃)₄Cl₃ + 2 S₂O₃^2-    2 Co ²⁺ + S₄O₆^2- + 8 NH₃ + 6 Cl ^–  

Molarity of S₂O₃^2- = 0.1500 M

Volume of S₂O₃^2- = 25 mL

Moles of S₂O₃^2- = 0.1500 * 0.025

= 0.00375 moles

From the equation:

2 moles sample reacts with 2 moles S₂O₃^2- .

Moles of sample required = 0.00375

Molar mass of Co(NH₃)₄Cl₃ = 233.41 g/mole

Mass of sample = 0.00375 * 233.41

= 0.875 g