Textbook Fundamentals qE.29

[Tala Ayoub 1H]

A chemist measured out 8.61 g of copper(II) chloride tetrahydrate.
(a) How many moles of copper(II) chloride tetrahydrate were measured out? (b) How many moles of Cl- ions are present in the sample? (c) How many
molecules are present in the sample? (d) What fraction of the total mass of the sample is due to oxygen?

How do I calculate the fraction of the total mass of the sample which is due to oxygen?

[Robert Nguyen 14B-3E]

Since the whole sample is the same molecule, the percent of oxygen in 8.61 grams would be the same percent of oxygen in one molecule. First we would find the mass of oxygen (CuCl2 * 4H20 -> 4 oxygens = 4 * 16), divide that by the total mass (CuCl2 * 4H20 = 63.546 + 2 * 35.45 + 4 * (2 * 1 + 16), and then multiply by 100% to get the percentage.