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Definite proportions and percent by mass

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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.

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% 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.

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% by mass

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Example What is the percent by mass of oxygen in water (H 2 O) – 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

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Example continued H 2 O 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

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Now time for the calculation

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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!

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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.

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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.

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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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