Homework - Practice 1-4 page 248 1. Fe3C = 93.31% Fe and 6.69% C 2. SO2 = 50.05% S and 49.95% O 3. NH4NO3 = 35.00% N, 5.05% H, and 59.96% O 4. a. SrBr2 = 35.41% Sr and 64.59% Br b. CaSO4 = 29.44% Ca, 23.55% S, and 47.01% O c. Mg(CN)2 = 31.83% Mg, 31.46 % C, and 36.70% N d. Pb(CH3COO)2 = 63.70% Pb, 14.77% C, 1.86% H and 19.67% O

7.3 Percent Composition The percentage by mass of EACH element in a compound

1st From the subscripts, determine the mass contributed by each element and add these to get molar mass. 2nd Divide the mass of EACH element by the molar mass. 3rd Multiply by 100 to find the percentage composition of that element.

Example: Determine the percent composition of lithium sulfate. 13.88g 100 = 12.62% Li 109.95 g Li2SO4 2 x 6.94 g Li = 13.88 g Li 1 x 32.07 g S = 32.07 g S + 4 x 16.00 g O = 64.00 g O molar mass = 109.95 g 32.07 g 100 = 29.17% S 109.95 g 64.00 g 100 = 58.21% O 109.95 g

Finding Mass from Percent Composition 1st Change EACH percent into a decimal by dividing by 100. 2nd Multiply EACH decimal number by total mass of substance.

Example: A 26. 2 gram sample of methanol was discovered to contain 23 Example: A 26.2 gram sample of methanol was discovered to contain 23.7% C, 15.6% H, and 60.7% O. What is the mass of each element?

23.7 = 0.237 100 x 26.2 g = 6.21 g C 15.6 = 0.156 100 x 26.2 g = 4.09 g H 60.7 = 0.607 100 x 26.2 g = 15.9 g O

11 g fat = 39% 28 g total