hc ch10 sec 3 - eastern regional high schoolweb.eccrsd.us/borda/public/honors/hc_ch10_3.pdf ·...

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10.3 Percent Composition and Chemical Formulas

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Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. .

Chapter 10 Chemical Quantities 10.1 The Mole: A Measurement of

Matter 10.2 Mole-Mass and Mole-Volume

Relationships 10.3 Percent Composition and

Chemical Formulas


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A tag sewn into the seam of a shirt usually tells you what fibers were used to make the cloth and the percent of each.

CHEMISTRY & YOU

What does the percent composition of a compound tell you?


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Percent Composition of a Compound

How do you calculate the percent composition of a compound?



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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.


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The percent composition of potassium chromate, K2CrO4, is:


K = 40.3% Cr = 26.8% O = 32.9%


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K = 40.3% Cr = 26.8%

+ O = 32.9% 100%

These percents must total 100%.


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These percents must total 100%. •  The percent composition of a compound is

always the same.


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If you know the relative masses of each element in a compound, you can calculate the percent composition of the compound.


Percent Composition from Mass Data

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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%.

Percent Composition from Mass Data


% by mass of element = mass of element

mass of compound × 100%


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When a 13.60-g sample of a compound containing only magnesium and oxygen is decomposed, 5.40 g of oxygen is obtained. What is the percent composition of this compound?

Sample Problem 10.9

Calculating Percent Composition from Mass Data


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

KNOWNS mass of compound = 13.60 g mass of oxygen = 5.40 g O mass of magnesium = 13.60 g – 5.40 g O = 8.20 g Mg

UNKNOWNS percent by mass of Mg = ?% Mg percent by mass of O = ?% O

Sample Problem 10.9

Analyze List the knowns and the unknowns. 1


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Determine the percent by mass of Mg in the compound.

Sample Problem 10.9

Calculate Solve for the unknowns. 2

% Mg = mass of Mg


= 60.3% Mg

= × 100% 8.20 g

13.60 g

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Determine the percent by mass of O in the compound.

Sample Problem 10.9


% O = mass of O


5.40 g

= 39.7% O

= × 100% 13.60 g


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The percents of the elements add up to 100%.

60.3% + 39.7% = 100%

Sample Problem 10.9

Evaluate Does the result make sense? 3


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You can also calculate the percent composition of a compound using its chemical formula.

Percent Composition from the Chemical Formula


•  The subscripts in the formula are used to calculate the mass of each element in a mole of that compound.

•  Using the individual masses of the elements and the molar mass, you can calculate the percent by mass of each element.


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You can also calculate the percent composition of a compound using its chemical formula.

Percent Composition from the Chemical Formula


% by mass of element

mass of element in 1 mol compound molar mass of compound

× 100% =

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Propane (C3H8), the fuel commonly used in gas grills, is one of the compounds obtained from petroleum. Calculate the percent composition of propane.

Sample Problem 10.10

Calculating Percent Composition from a Formula


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Calculate the percent by mass of each element by dividing the mass of that element in one mole of the compound by the molar mass of the compound and multiplying by 100%. KNOWNS mass of C in 1 mol C3H8 = 3 mol × 12.0 g/mol = 36.0 g mass of H in 1 mol C3H8 = 8 mol × 1.0 g/mol = 8.0 g molar mass of C3H8 = 36.0 g/mol + 8.0 g/mol = 44.0 g/mol UNKNOWNS percent by mass of C = ?% C percent by mass of H = ?% H


Analyze List the knowns and the unknowns. 1


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Determine the percent by mass of C in C3H8.

% C = mass of C in 1 mol C3H8

molar mass of C3H8 × 100%

= 81.8% C

= × 100% 36.0 g 44.0 g




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Determine the percent by mass of H in C3H8.

% H = mass of H in 1 mol C3H8

molar mass of C3H8 × 100%

= 18% H

= × 100% 8.0 g

44.0 g



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The percents of the elements add up to 100% when the answers are expressed to two significant figures (82% + 18% = 100%).




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

•  To do this, multiply the mass of the compound by a conversion factor based on the percent composition of the element in the compound.

Percent Composition as a Conversion Factor



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Propane is 81.8% carbon and 18% hydrogen.

•  You can use the following conversion factors to solve for the mass of carbon or hydrogen contained in a specific amount of propane.

Percent Composition as a Conversion Factor


81.8 g C 100 g C3H8

and 18 g H

100 g C3H8


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What information can you get from the percent composition of a compound?

CHEMISTRY & YOU

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What information can you get from the percent composition of a compound?

CHEMISTRY & YOU

You can use percent composition to determine the mass of an element in a sample of a compound of a given size. You can also determine the empirical formula of the compound.


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Calculate the mass of carbon and the mass of hydrogen in 82.0 g of propane (C3H8).


Calculating the Mass of an Element in a Compound Using Percent Composition


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Use the conversion factors based on the percent composition of propane to make the following conversions: grams C3H8 → grams C and grams C3H8 → grams H.

KNOWN mass of C3H8 = 82.0 g


Analyze List the known and the unknowns. 1

UNKNOWNS mass of carbon = ? g C mass of hydrogen = ? g H


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•  To calculate the mass of C, first write the conversion factor to convert from mass of C3H8 to mass of C.

81.8 g C 100 g C3H8

From Sample Problem 10.10, the percent by mass of C in C3H8 is 81.8%.



•  Multiply the mass of C3H8 by the conversion factor.

81.8 g C 100 g C3H8

82.0 g C3H8 × = 67.1 g C

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•  To calculate the mass of H, first write the conversion factor to convert from mass of C3H8 to mass of H. From Sample

Problem 10.10, the percent by mass of H in C3H8 is 18%.



•  Multiply the mass of C3H8 by the conversion factor.

18 g H 100 g C3H8

82.0 g C3H8 × = 15 g H

18 g H 100 g C3H8


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The sum of the two masses equals 82 g, the sample size, to two significant figures (67 g C + 15 g H = 82 g C3H8).



hc ch10 sec 3 - eastern regional high schoolweb.eccrsd.us/borda/public/honors/hc_ch10_3.pdf ·...

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