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chemistry

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Percent Composition and Chemical Formulas

> The Percent Composition of a Compound

The percent by mass of an element in a compound is the number of grams of the element divided by the mass in grams of the compound, multiplied by 100%.

10.3

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Percent Composition and Chemical Formulas

> The Percent Composition of a Compound

Percent Composition from Mass Data

The relative amounts of the elements in a compound are expressed as the percent composition or the percent by mass of each element in the compound.

10.3

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SAMPLE PROBLEM

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10.9

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Practice Problems for Sample Problem 10.9

Problem Solving 10.33 Solve Problem 33 with the help of an interactive guided tutorial.

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Percent Composition and Chemical Formulas

> The Percent Composition of a Compound

Percent Composition from the Chemical Formula

10.3

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SAMPLE PROBLEM

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10.10

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Practice Problems for Sample Problem 10.10

Problem Solving 10.34 Solve Problem 34 with the help of an interactive guided tutorial.

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Percent Composition and Chemical Formulas

> The Percent Composition of a Compound

Percent Composition as a Conversion Factor

You can use percent composition to calculate the number of grams of any element in a specific mass of a compound.

10.3

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Percent Composition and Chemical Formulas

> The Percent Composition of a Compound

Propane (C3H8) is 81.8% carbon and 18% hydrogen. You can calculate the mass of carbon and the mass of hydrogen in an 82.0 g sample of C3H8.

10.3

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Percent Composition and Chemical Formulas

> Empirical Formulas

The empirical formula gives the lowest whole-number ratio of the atoms of the elements in a compound.

The empirical formula of a compound shows the smallest whole-number ratio of the atoms in the compound.

10.3

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Percent Composition and Chemical Formulas

> Empirical Formulas

Ethyne (C2H2) is a gas used in welder’s torches. Styrene (C8H8) is used in making polystyrene.

These two compounds of carbon have the same empirical formula (CH) but different molecular formulas.

10.3

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Percent Composition and Chemical Formulas

> Empirical Formulas

You can take percent composition and use it do determine the empirical formula of a compound as follows:

•Assume you have 100g of the compound.•With 100 g, you can change the percents to masses•Convert the masses to moles.•Find the smallest number of moles.•Divide each mole value by the smallest number of moles.•The values of the ratios will give you the subscripts of the empirical formula•Watch out for fractions 0.33 is 1/3 – multiply by 3.

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SAMPLE PROBLEM

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10.11

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SAMPLE PROBLEM

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SAMPLE PROBLEM

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Practice Problems for Sample Problem 10.11

Problem-Solving 10.37 Solve Problem 37 with the help of an interactive guided tutorial.

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Practice Problems

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Practice Problems

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Percent Composition and Chemical Formulas

> Molecular Formulas

The molecular formula of a compound is either the same as its experimentally determined empirical formula, or it is a simple whole-number multiple of its empirical formula.

10.3

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Percent Composition and Chemical Formulas

> Molecular Formulas

When given the empirical formula and the molar mass,

•determine the molar mass for the empirical formula, • then divide the molar masses• the whole number answer is then multiplied by the empirical formula.

Xformula empirical theof massmolar formulamolecular theof massmolar

formulamolecular formula) lX(empirica

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Percent Composition and Chemical Formulas

> Molecular Formulas10.3

Methanal, ethanoic acid, and glucose all have the same empirical formula—CH2O.

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Percent Composition and Chemical Formulas

> Molecular Formulas10.3

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SAMPLE PROBLEM

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10.12

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Practice Problems for Sample Problem 10.12

Problem-Solving 10.38 Solve Problem 38 with the help of an interactive guided tutorial.

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Practice Problems

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Section Quiz

-or-Continue to: Launch:

Assess students’ understanding of the concepts in Section

10.3 Section Quiz.

10.3.

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1. Calculate the percent by mass of carbon in cadaverine, C5H14N2, a compound present in rotting meat.

a. 67.4% C

b. 58.8% C

c. 51.7% C

d. 68.2% C

10.3 Section Quiz.

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2. Which of the following is NOT an empirical formula?

a. NO2

b. H2N

c. CH

d. C3H6

10.3 Section Quiz.

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10.3 Section Quiz.

3. Determine the molecular formula of a compound that contains 40.0 percent C, 6.71 percent H, and 53.29 percent O and has a molar mass of 60.05 g.

a. C2H4O2

b. CH2O

c. C2H3O

d. C2H4O

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Percent Composition and Chemical Formulas

> Concept Map 10

Concept Map 10 Solve the Concept Map with the help of an interactive tutorial.

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