A salt containing chromium, chlorine, and water has the formula CrCln(H2O)m. — Stoichiometry / Electrochemistry / Coordination Chemistry Chemistry Question
Problem Context
A salt containing chromium, chlorine, and water has the formula CrCln(H2O)m.
A sample of the salt is electrolyzed for 1310 s using a current of 1.24 A and deposits 0.292 g metallic Cr. What is the value of n in the salt?
Model Answer
(1310 s)(1.24 A)/(96485 C/mol) = 0.0168 mol electrons
0.292 g Cr/(52.00 g/mol) = 5.62 × 10-3 mol Cr
0.0168 mol electrons/(5.62 × 10-3 mol Cr) = 2.99 mol electrons/mol Cr
Thus Cr must be in the +3 oxidation state, n = 3.
A 3.000 g sample of the chromium salt is heated carefully at 600 ºC to drive off any water in the salt, until the sample achieves a constant mass of 1.783 g. What is the value of m in the salt?
Model Answer
3.000 g – 1.783 g = 1.217 g H2O/(18.02 g/mol) = 0.06754 mol H2O
1.783 g CrCl3/(158.35 g/mol) = 0.01126 mol CrCl3
m = 0.06754 mol H2O/0.01126 mol CrCl3 = 6
A 0.300 g sample of the chromium salt is dissolved in 10 mL water to which a few drops of Na2CrO4 solution have been added. A 0.400 M solution of silver nitrate is titrated quickly into the solution until the appearance of a dark red color; this requires 2.81 mL of the AgNO3 solution.
i. Write balanced chemical equations for the reaction taking place during the titration and the reaction that takes place at the endpoint.
Model Answer
During the titration, Ag+ (aq) + Cl– (aq) → AgCl (s)
At the endpoint, 2 Ag+ (aq) + CrO4 2- (aq) → Ag2CrO4 (s)
ii. How many moles of chloride are detected in this titration per mole of chromium present?
Model Answer
Mol Cr salt = 0.3000 g/(266.47 g/mol) = 1.126 × 10-3 mol
Mol Ag+ added = mol Cl– present = (0.400 mol/L) × (2.81 × 10-3 L) = 1.12 × 10-3 mol
There is thus 1.00 mol Cl– titrated per mol Cr.
Propose an explanation for the result in part (c)( ii).
Model Answer
Presumably two of the chlorides are bonded directly to the Cr(III) ion and therefore do not react rapidly with Ag+ (aq); the salt is likely best formulated [CrCl2(H2O)4]Cl•2 H2O.