THURSDAY, 19TH SEPTEMBER 2013

•Today in Chemistry• Percent Composition

•Assignment•Read pp. 320-355

KEY IDEA

•Formula mass = The sum of the atomic masses in the formula. KNO3

K = 39.1 amuN = 14.0 amuO = 3 x 16.0 amuKNO3 101.1 amu

AMU?Atomic Mass Unit 39.1 g of K is equal to 14.0 g of N (same # of particles)

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.

KEY IDEA

•Percent Composition~Remember it adds up to 100% Percent Composition by Mass% Comp. = # Atoms x Atomic Mass x

100Formula Mass

% H in H2O = 2 atoms H x 1.0 amu x 10018.0 amu H2O

% H in H2O = 11 % So…. %O = ?

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KEY IDEA

• The Law of Multiple ProportionsWhen different compounds are formed by the same elements, different masses combine in simple whole number ratios. Methane CH4 Ethane C2H6

FM = 16.0 FM = 30 % H = 25% % H = 20%

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