Question

a) Copper solutions are typically blue to blue green in color. What color of light would they be expected to absorb in the greatest amount?

		Red
		Violet
		Blue
		Green

b) Based on your answer to the previous question (hopefully you have the correct answer), what wavelength range would you expect to see the maximum absorption of light (in the visible range)?

		500 nm to 600 nm
		400 nm to 500 nm
		300 nm to 400 nm
		600 nm to 700 nm

c) Given the information that the tartrate ion is the conjugate base of a weak acid, how do you expect the solubility of copper (II) tartrate to be affected by pH?

		decreased with lower pH.
		increased with lower pH.
		most soluble at pH = 7
		no change in solubility with pH.

d) It was an intention to have a common ion experiment with the lab, but it did not work when we tested the experiment. What would the solubility of copper (II) tartrate be in a solution that contains 0.19 M sodium tartrate? (Use the literature for copper (II) tartrate K_sp = 4.0 x 10^-4) Please enter answer in standard notation.

e) Your value for K_sp will not be the same as the literature. It may not even be close. It is likely to be larger than the literature value. What is one logical reason your value may be larger?

		evaporation of water leads to a more concentrated solution with greater solubility.
		the accuracy of our spectrometers leads to lower than expected copper concentration readings.
		the gravity filtration used might not be very effective at removing small undissolved solid particles from the saturated solution.
		students just cannot get results that are as good as trained experts.

Answer 1

a.) RED

Reason : If a particular colour is absorbed from white light, what your eye detects by mixing up all the other wavelengths of light is its complementary colour. Copper(II) sulphate solution is pale blue (cyan) because it absorbs light in the red region of the spectrum. Cyan is the complementary colour of red.

b.) 600 nm to 700 nm

Reason : Beacuse red light comes in region 600 nm to 700 nm

c.) increased with lower pH.

Reason : conjugate base of a weak acid is a strong base. Hence, solubility of copper (II) tartrate increased with lower pH. Common ion effect.

d.) Copper tartarate : CuC₄H₄O₆

There is one mole of Cu2+ and one mole of tartarate in copper tartarate.

So,

K_sp   = s²

Ksp = 4.0 x 10^-4 = s²( given)

s = 2.0 X 10^-2 mol/L

Hence, solubility =  2.0 X 10^-2 mol/L

e.) evaporation of water leads to a more concentrated solution with greater solubility.