Question

5. Calculate the mass of oxý gen in a 69.5 g sample of aluminum oxide. 6. How many total atoms are in a 75 g sample of Ag:0 7. (2 pts) Calculate the percent by mass of a solution that is 4.60 M NaCI and has a density of 1.21 g/mL 8. (2 bonus points) A 16.02 g sample of C,H,O, compound undergoes combustion and yields 24.02 g of carbon dioxide and 6.56 g of water. (All the carbon in the compound is used to make carbon dioxide, and all the hydrogen in the compound is used to make water.) The molar mass of the molecular compound is 176.1 g. What are the empirical and molecular formulas for the compound? (e) 24.02, CO Chapter 9 Mole Quantities

Answer 1

Solution )

5. We know that the molar mass of aluminium is 26.98 g/mol and that of oxygen is 16.00 g/mol.

So,1 mole of Al₂O₃ therefore has a mass of 2(26.98) + 3(16.00) = 101.96 g.
Divide this into the given mass, to find the number of moles here:
(69.5g)/(101.96 g/mol) = 5.811 mol.
There are 3 moles of oxygen per mole of aluminium oxide, so there are
3(5.811) = 17.433 moles of aluminium in the sample.
Then there are (17.433 mol)((16.00 g/mol) = 278.93 g of O.