The question
Name: ______________________ Date: ____________ The formula of a copper oxide from heating data (10 points)
Experimental setup
Heating copper in a crucible
A student places pure copper powder in a clean, dry crucible of known mass and heats it strongly in air. The copper reacts with oxygen from the air to form a copper oxide. After cooling, the student weighs the crucible and its contents.
| Measurement | Mass (g) |
|---|---|
| Empty crucible | 21.304 |
| Crucible + copper before heating | 22.575 |
| Crucible + copper oxide after heating and cooling | 22.735 |
(a) Calculate the number of moles of copper used in Trial 1. (Cu 63.55 g/mol) [1 point]
(b) Calculate the number of moles of oxygen atoms that combined with the copper. [1 point]
(c) Determine the empirical formula of the copper oxide formed in Trial 1. Show your reasoning. [1 point]
(d) Calculate the mass percent of copper in the product of Trial 1. [1 point]
(e) Natural copper has two isotopes: copper-63 (62.930 amu, 69.15%) and copper-65 (64.928 amu, 30.85%). (i) Calculate the average atomic mass of copper. (ii) Describe the mass spectrum of a pure copper sample: the number of peaks, where they are, and their relative heights. [2 points]
(f) Suppose some of the copper in Trial 1 did not react with oxygen. Would the calculated ratio of moles of O to moles of Cu be too high, too low or unchanged? Justify your answer. [2 points]
(g) In Trial 2, the student uses the same mass of copper but heats it much longer, stirring it, in a stream of pure oxygen. The crucible and product now have a mass of 22.895 g. Determine the empirical formula of the Trial 2 product, and explain how the results of the two trials are consistent with the law of definite proportions. [2 points]