[12%] Copper(II) sulfate pentahydrate, CuSO4 • 5H2O, is a blue crystalline solid. Upon gentle heatin — Hydrates, Spectroscopy, Titration Chemistry Question
Problem Context
[12%] Copper(II) sulfate pentahydrate, CuSO4 • 5H2O, is a blue crystalline solid. Upon gentle heating, it loses water to form anhydrous CuSO4, which is a white solid.
When 5.000 g CuSO4 • 5H2O is heated to remove all of its water, what mass of anhydrous CuSO4 will be produced?
Model Answer
M of CuSO4 • 5 H2O = 249.70, M of CuSO4 = 159.62
(5.00 g CuSO4 • 5 H2O)/249.70 g mol-1 = 2.002 × 10-2 mol CuSO4 • 5 H2O
2.002 × 10-2 mol × 159.62 g mol-1 = 3.196 g anhydrous CuSO4
Explain the change in color on dehydration of CuSO4 • 5H2O.
Model Answer
Copper(II) is d9 and is expected to show (weak) absorption of light regardless of its environment. Dehydration of copper(II) sulfate results in a change of ligands from water ligands to sulfate ligands. The blue color is due to absorption of red light at the extreme low-energy end of the visible spectrum. The change to the (weaker-field) sulfate ligands reduces the energy of the absorption into the near-infrared, so that visible light is not absorbed at all. This makes the solid appear white.
A solution of 0.0506 g Cu(CH3COO)2 • H2O is made up with water to a volume of 5.00 mL water. This solution in a 1-cm cuvette produces the visible spectrum shown. What wavelength would be the best choice to determine the concentration of Cu(II)?
Model Answer
The maximum absorption of light occurs at 763 nm, so that is the most sensitive wavelength to measure.
A student determines the value of n in an unknown crystalline hydrate of copper(II) nitrate, Cu(NO3)2 • n H2O, by preparing 5.00 mL of an aqueous solution of a known mass of the compound and measuring its absorbance at the wavelength determined in part c. However, the student inserts the cuvette into the spectrophotometer without first wiping fingerprints from it. How will this affect the value of n determined in the experiment?
Model Answer
The fingerprint will scatter light, increasing the apparent absorbance and thus the apparent amount of copper. This will mean that less mass will be ascribed to water, giving a smaller value of n.
An alternative method for determining the degree of hydration of the copper nitrate is to allow a known mass of compound to react with excess KI solution, which produces a yellow-brown suspension. Write a balanced net ionic equation for this reaction.
Model Answer
2 Cu2+(aq) + 5 I–(aq) → 2 CuI(s) + I3–(aq)
The experiment in part e is carried out with 0.1000 g of the hydrated copper(II) nitrate. To the resulting mixture is added a 0.0250 M solution of sodium thiosulfate, Na2S2O3, until the color of the mixture has just dissipated, leaving a milky white suspension. This requires 17.20 mL of added sodium thiosulfate solution. What is the value of n for the Cu(NO3)2 • n H2O?
Model Answer
The thiosulfate reacts with triiodide to form tetrathionate and iodide in the following balanced equation:
2 S2O3 2-(aq) + I3 –(aq) → S4O6 2-(aq) + 3 I–(aq)
Since each 2 mol of Cu2+ produced one mol I3 –, each mol of S2O3 2- corresponds to one mol of Cu2+:
(0.01720 L S2O3 2-) × (0.0250 mol L-1) = 4.30 × 10-4 mol S2O3 2- = 4.30 × 10-4 mol Cu2+
{4.30 × 10-4 mol Cu(NO3)2} × (187.57 g mol-1) = 0.0807 g Cu(NO3)2
0.1000 g hydrated Cu(NO3)2 – 0.0807 g anhyd. Cu(NO3)2 = 0.0193 g H2O
(0.0193 g H2O)/(18.02 g mol-1) = 1.07 × 10-3 mol H2O
n = (1.07 × 10-3 mol H2O)/(4.30 × 10-4 mol Cu) = 2.50