A strip of electrolytically purified copper weighing 3.178 grams is strongly heated in a stream of oxygen until it is all converted to a black oxide. The resultant black powder weighs 3.978 g. What is the formula of the oxide?

Question

Quizplus · Accepted Answer

The increase in mass from 3.178 g to 3.978 g is due to the oxygen that has combined with the copper. The mass of oxygen added is 3.978 g - 3.178 g = 0.800 g. The molar mass of copper is approximately 63.55 g/mol, and the molar mass of oxygen is approximately 16.00 g/mol. The moles of copper are 3.178 g / 63.55 g/mol ≈ 0.0500 mol, and the moles of oxygen are 0.800 g / 16.00 g/mol = 0.0500 mol. Since the moles of copper and oxygen are equal, the simplest formula is CuO.

A Strip of Electrolytically Purified Copper Weighing 3