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Percent Composition. Parts of a Whole. Percent. Short for per centum …”by the hundred” Part of a whole If there are 10 people in a room and 4 of them have blue eyes, what percentage of people have blue eyes?. Composition. Chemical formula Molar mass. Percent Composition.

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## Percent Composition

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**Percent Composition**Parts of a Whole**Percent**• Short for per centum…”by the hundred” • Part of a whole • If there are 10 people in a room and 4 of them have blue eyes, what percentage of people have blue eyes?**Composition**• Chemical formula • Molar mass**Percent Composition**• Write the formula • Find the molar mass • Determine the percentage of the part in question**Problem #1**• Determine the percent composition of water.**Problem #1—Step 1**• Write its formula • H2O**Problem #1—Step 2**• Determine its molar mass • H—2 x 1.01 = 2.02 • O—1 x 16.00 = 16.00 18.02 g/mol**Problem #1—Step 3**• Determine the portion that each element makes up by its mass and multiply by 100…determine the percent composition • H—2 x 1.01 = 2.02 x 100 = 11.2% H 18.02 • O—1 x 16.00 = 16.00 x 100 = 88.8% O 18.02**Problem #2**• Determine the percent composition of aluminum carbonate.**Problem #2—Step 1**• Write its formula • Al2(CO3)3**Problem #2—Step 2**• Determine its molar mass • Al—2 x 26.98 = 53.96 • C—3 x 12.01 = 36.03 • O—9 x 16.00 = 144.00 233.99 g/mol**Problem #1—Step 3**• Determine the percent composition • Al—2 x 26.98 = 53.96 x 100 = 23.1% Al 233.99 • C—3 x 12.01 = 36.03 x 100 =15.4% C 233.99 • O—9 x 16.00 = 144.00 x 100 =61.5% O 233.99**Problem #3**• A compound contains only copper and chloride ions. It is found to be 47.3% copper. Determine the formula of the compound.**Problem #3—Step 1**• Determine the possible formulas • Copper can have charges of 1+ or 2+ • So, your choices are: CuCl and CuCl2**Problem #3—Step 2**• Determine both molar masses • Cu—1 x 63.55 = 63.55 • Cl—1 x 35.45 = 35.45 99.00 g/mol • Cu—1 x 63.55 = 63.55 • Cl—2 x 35.45 = 70.90 134.45 g/mol**Problem #3—Step 3**• Determine the percentage of copper in each. • Cu—1 x 63.55 = 63.55 x 100 = 64.2% Cu 99.00 • Cu—1 x 63.55 = 63.55 x 100 = 47.3% Cu 134.45 So, the answer is CuCl2**Problem #4**• A compound is known to contain 41.7% magnesium. Determine if the compound is magnesium fluoride, magnesium hydroxide, or magnesium oxide.**Problem #4—Step 1**• Determine the formulas • MgF2 • Mg(OH)2 • MgO**Problem #4—Step 2**• Determine all three molar masses • Mg—1 x 24.31 = 24.31 • F—2 x 19.00 = 38.00 62.31 g/mol • Mg—1 x 24.31 = 24.31 • O—2 x 16.00 = 32.00 • H—2 x 1.01 = 2.02 58.33 g/mol • Mg—1 x 24.31 = 24.31 • O—1 x 16.00 = 16.00 40.31 g/mol**Problem #4—Step 4**• Determine the percentage of magnesium in each. • Mg—1 x 24.31 = 24.31 x 100 =39.0% Mg 62.31 • Mg—1 x 24.31 = 24.31 x 100 = 41.7% Mg 58.33 • Mg—1 x 24.31 = 24.31 x 100 =60.3% Mg 40.31**Problem #4—Answer**• Mg(OH)2

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