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Laws of Chemical Proportions and Composition

Understand the Law of Definite Proportions and Law of Multiple Proportions in chemistry. Learn how to calculate percent composition and ratios, and solve practice problems to strengthen your understanding.

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Laws of Chemical Proportions and Composition

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  1. Chapter 3 Matter-Properties and Changes

  2. Outline Chapter 3 • Be sure to include all vocab words

  3. Law of Definite Proportions • The law states that regardless of the amount, a compound is always composed of the same elements in the same proportion by mass. • Percent by mass (Percent composition) • Shows the ratio of the mass of each element to the total mass of the compound as a percentage • Percent by mass = mass of element / mass of compound x 100

  4. Percent composition • Percent by mass of every element involved

  5. Ex. Pg 75 • Practice problems pg. 76 #20-24

  6. Law of Multiple Proportions • The law states that when different compounds are formed by a combination of the same elements, different masses of one element combine with the same relative mass of the other element in a ratio of small whole numbers.

  7. Law of Multiple Proportions • Cu+1 + Cl-1 = CuCl • Cu+2 + Cl-1 = CuCl2 • Quick lesson • What type of bond is this? • Ionic – opposite charges attract • Does the subscript 2 mean that the chlorine atoms are combined? • No (Show why) • + - - or - + - • Why does this type of reaction occur? • Metals tend to lose electrons but they can lose different numbers of electrons

  8. Law of Multiple Proportions • Cu+1 + Cl-1 = CuCl • Cu+2 + Cl-1 = CuCl2 • Use table 3-6 • Calculate the ratio of Cl per fixed mass of Cu. • What if you are only given %? • Always use a total of 100 g. • What if you are given mass? • Don’t worry about doing percentages • Always put the biggest number on top when getting whole number ratio.

  9. You have 100 g of CuCl and 40 g of CuCl. • What law is shown when the percent composition is calculated? • You have 10 g of C and 20 g of O in compound I. You have 10 g of C and 10 g of O in compound II. Calculate the ratio of O per fixed mass of C. • What law is shown when the ratio is calculated?

  10. Practice problem pg 77 #30

  11. Homework • Pg 83 60-69

  12. Math • x/y ÷ a/b = xb/ya

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