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© Copyright Pearson Prentice Hall

Percent Composition and Chemical Formulas

> The Percent Composition of a Compound

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%.

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I. Percent Composition from Mass Data

10.3


SAMPLE PROBLEM

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10.9


SAMPLE PROBLEM

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Practice Problems for Sample Problem 10.9

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II. Percent Composition from the Chemical Formula

10.3


SAMPLE PROBLEM

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10.10


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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.

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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.

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> Empirical Formulas

Empirical Formulas

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

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> 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.

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Rules for Determining Empirical Formula:

1.Convert grams of each element to moles

2.Divide each # of moles by the smallest # of moles

3.If answers to #2 are not all whole numbers, multiply by integers (2, 3…) to get whole numbers


SAMPLE PROBLEM

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10.11


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Practice Problems



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Molecular Formulas

Molecular Formulas

How does the molecular formula of a compound compare with the empirical formula?

10.3


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> Molecular 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.

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> Molecular Formulas10.3

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

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> Molecular Formulas10.3

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Steps to Determining the Molecular Formula of a Compound:

1.Determine the empirical formula

2.Find the molar mass of the empirical formula

3.Divide the molecular formula mass (given in problem) by the empirical formula mass (from step 2)

4.Multiply this integer by the empirical formula


SAMPLE PROBLEM

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10.12


SAMPLE PROBLEM

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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.


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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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> Concept Map 10

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

END OF SHOW

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Documents

empirical formula of

molar mass of glucose

molar mass of methanal

potassium chromate k2cro4

ethanoic acid acetic

methanal formaldehyde

styrene c8h8

ethyne c2h2