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Definite proportions and percent by mass

Definite proportions and percent by mass. Law of definite proportions. The Law of Definite Proportions states that a compound is always composed of the same elements in the same proportion by mass, no matter how large or small the sample.

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Definite proportions and percent by mass

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  1. Definite proportions and percent by mass

  2. Law of definite proportions • The Law of Definite Proportions states that a compound is always composed of the same elements in the same proportion by mass, no matter how large or small the sample. • The mass of the compound is equal to the sum of the masses of the elements that make up the compound.

  3. % by mass • Relative amounts of the elements in a compound can be expressed as percent by mass. • The percent by mass is the ratio of the mass of each element to the total mass of the compound expressed as a percentage.

  4. % by mass

  5. Example • What is the percent by mass of oxygen in water (H2O) • Step 1: determine mass of 1 hydrogen atom • Step 2: determine mass of 1 oxygen atom • Step 3: Multiply the mass of the corresponding element times the number of atoms indicated by subscripts in the formula of the compound • Step 4: Add the total masses of each element together (this gives the mass of the compound) • Step 5: Use the formula to calculate the % hydrogen and the % oxygen in the compound

  6. Example continued • H2O contains 2 atoms of hydrogen bonded to 1 atom of oxygen. • Mass of hydrogen=1.00794 u • Mass of oxygen=15.999 u • 1.00794 u x2 = 2.01588 u • 15.999 u x1 = 15.999 u • Total mass of compound = 18.04488 u

  7. Now time for the calculation • %H in H2O = • %O in H2O = O • The total of all percentages should equal 100%! NEVER MORE, NEVER LESS!

  8. Another example • You may be required to work backwards, given a percent by mass (or percent composition) and asked to provide the chemical formula. • We will do this in this chapter, and again at another point in the course… So pay close attention!

  9. Let’s try one like this! • A compound has a percent composition of 42.1% carbon, 6.5% hydrogen, and 51.4% oxygen. The compounds total mass is 342.30008 atomic mass units. What is the formula for this compund? • This will be worked out on board.

  10. But, what if we’re not given the mass of the compound? • A compound has a percent composition of 36.1% lithium and 63.9% chlorine. What is the chemical formula for this substance? • Will be done on board.

  11. Let’s try one more, fairly simple one • 1.0 g of hydrogen reacts completely with 19.0 g of fluorine. What is the percent by mass of hydrogen in the compound that is formed? What about the percent by mass of fluorine? • Again, this is done on the board.

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