1. THURSDAY, 19 TH SEPTEMBER 2013 Today in Chemistry Percent Composition Assignment Read pp. 320-355

2. KEY IDEA Formula mass = The sum of the atomic masses in the formula. KNO 3 K = 39.1 amu N = 14.0 amu O = 3 x 16.0 amu KNO 3 101.1 amu AMU ? Atomic Mass Unit 39.1 g of K is equal to 14.0 g of N (same # of particles)

3. KEY IDEA The law of Definite Proportions A compound is always composed of the same elements in the same proportion by mass, no matter how large or small the sample.

4. KEY IDEA Percent Composition ~Remember it adds up to 100% Percent Composition by Mass % Comp. = # Atoms x Atomic Mass x 100 Formula Mass % H in H 2 O = 2 atoms H x 1.0 amu x 100 18.0 amu H 2 O % H in H 2 O = 11 % So…. %O = ? Page 87

5. KEY IDEA The Law of Multiple Proportions When different compounds are formed by the same elements, different masses combine in simple whole number ratios. Methane CH 4 Ethane C 2 H 6 FM = 16.0 FM = 30 % H = 25% % H = 20% Page 89