2. Objectives Calculate the formula mass or molar mass of any given compound. Use molar mass to convert between mass in grams and amount in moles of a chemical compound. Calculate the number of molecules, formula units, or ions in a given molar amount of a chemical compound. Calculate the percentage composition of a given chemical compound.
9. Molar Mass as a conversion Amount of compound (moles) Mass of a compound (grams) Number of molecules or formula units Molar mass 6.02 x 1023 ÷ x 6.02 x 10 23 Molar mass ÷ x
10.
11. How many molecules of ibuprofen are in the bottle? 6.022 x 1023 molecules o.16 mol = 9.6 x 1022 molecules X 1 mol More Practice problems How many moles of the compound are there in 6.60 g (NH4)2SO4? How many molecules are there in 25.0 g H2SO4? What is the mass in grams of 6.25 mol of copper(II) nitrate Answer: 0.0500 mol Answer: 1.53 x 1023 molecules Answer: 1170 g
12. Percentage composition Useful in determining the percent by mass of each element in a compound 1. Find the percent composition of copper(I) sulfide, Cu2 S. First: find the total molar mass Cu 2 x 63.55 g = 127.1 g Cu S 1 x 32.07 g = 32.07 g S 159.2 g Cu2S Second: find the percent of each 127.1 g Cu X 100 = 79.84 % Cu 159.2 g Cu2S 32.07 g S X 100 = 20.14 % S 159.2 g Cu2S
13. More Practice: percent composition Find the mass percentage of water in sodium carbonate decahydrate, Na2CO3 10H2O, which as a molar mass of 286.14 g/mol. Find the percentage composition of PbCl2 Ba(NO3)2 10 mol H2O x 18.02 g H2O 1 mol H2O = 180.2 g H2O 180.2 g H2O x 100 = 62.98 % 286.14 g Na2CO3 10 H2O Answer: 74.51 % Pb, 25.49 % Cl Answer: 52.55 % Ba, 10.72 % N, 36.73 % O
