The question
Name: ______________________ Date: ____________ Making and measuring copper(II) solutions (10 points)
Data table
Copper(II) nitrate standards
A student needs a set of copper(II) nitrate, Cu(NO₃)₂ (187.56 g/mol), standards to find the copper(II) concentration of a pool-cleaning solution. She first makes 100.0 mL of 0.200 M Cu(NO₃)₂ stock, then dilutes portions of it to make the standards below and measures their absorbance at 810 nm in a 1.00 cm cuvette, after zeroing the spectrophotometer with water. The pool-cleaning solution gives an absorbance of 0.413.
| [Cu²⁺] (M) | Absorbance |
|---|---|
| 0.0100 | 0.119 |
| 0.0200 | 0.235 |
| 0.0300 | 0.356 |
| 0.0400 | 0.471 |
| 0.0500 | 0.592 |
(a) Calculate the mass of Cu(NO₃)₂ needed to make 100.0 mL of 0.200 M stock. [1 point]
(b) Describe how the student should prepare the stock solution, naming the equipment. [2 points]
(c) Calculate the volume of 0.200 M stock needed to make 50.00 mL of the 0.0400 M standard. [1 point]
(d) In the box on your answer sheet, draw a particle diagram of the solution showing one Cu²⁺ ion, the correct number of NO₃⁻ ions, and at least two water molecules around one cation and one anion. [2 points]
(e) On the grid on your answer sheet, plot the absorbance of the standards against concentration and draw a best-fit line. [1 point]
(f) Calculate the concentration of Cu²⁺ in the pool-cleaning solution. [1 point]
(g) Water drops were left on the outside of the cuvette when the pool solution was measured, but not for the standards. Predict whether the calculated Cu²⁺ concentration is too high, too low or unaffected, and justify your prediction. [2 points]