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Chapter 6Chemical Quantities

6.4

Calculations Using Molar Mass

Copyright © 2008 by Pearson Education, Inc.Publishing as Benjamin Cummings

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Molar mass conversion factors • Are written from molar mass.• Relate grams and moles of an element or

compound.

Example: Write molar mass factors for methane CH4

used in gas cook tops and gas heaters.

Molar mass:

1 mol CH4 = 16.04 g

Conversion factors:

16.04 g CH4 and 1 mol CH4

1 mol CH4 16.04 g CH4

Molar Mass Factors

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Acetic acid C2H4O2 gives the sour taste to vinegar. Write two molar mass conversion factors for acetic acid.

Learning Check

Copyright © 2008 by Pearson Education, Inc.Publishing as Benjamin Cummings

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Acetic acid C2H4O2 gives the sour taste to vinegar.Write two molar mass factors for acetic acid.

Calculate molar mass:

24.02 + 4.032 = 32.00 = 60.05 g/mol

1 mol of acetic acid = 60.05 g acetic acid

Molar mass factors

1 mol acetic acid and 60.05 g acetic acid 60.05 g acetic acid 1 mol acetic acid

Solution

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Molar mass factors are used to convert between the grams of a substance and the number of moles.

Calculations Using Molar Mass

Grams Molar mass factor Moles

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Aluminum is used to build lightweight bicycleframes. How many grams of Al are 3.00 mol Al?

Molar mass equality: 1 mol Al = 26.98 g Al

Setup with molar mass as a factor:

3.00 mol Al x 26.98 g Al = 80.9 g Al1 mol Al

molar mass factor for Al

Moles to Grams

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Learning CheckAllyl sulfide C6H10S is a compound that has the odor of garlic. How many moles of C6H10S are in 225 g C6H10S?

Copyright © 2008 by Pearson Education, Inc.Publishing as Benjamin Cummings

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Calculate the molar mass of C6H10S.

(6 x 12.01) + (10 x 1.008) + (1 x 32.07)

= 114.21 g/mol

Set up the calculation using a mole factor.

225 g C6H10S x 1 mol C6H10S

114.21 g C6H10S

molar mass factor(inverted)

= 1.97 mol C6H10S

Solution

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Grams, Moles, and Particles

A molar mass factor and Avogadro’s number

convert• Grams to particles molar mass Avogadro’s

number

(g mol particles)• Particles to grams

Avogadro’s molar mass number

(particles mol g)

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Learning Check

How many H2O molecules are in 24.0 g H2O?

1) 4.52 x 1023

2) 1.44 x 1025

3) 8.02 x 1023

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Solution

How many H2O molecules are in 24.0 g H2O?

3) 8.02 x 1023

24.0 g H2O x 1 mol H2O x 6.022 x 1023 H2O molecules

18.02 g H2O 1 mol H2O

= 8.02 x 1023 H2O molecules

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Learning Check

If the odor of C6H10S can be detected from 2 x 10-13 g in one liter of air, how many molecules of C6H10S are present?

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SolutionIf the odor of C6H10S can be detected from

2 x 10-13 g in one liter of air, how many molecules

of C6H10S are present?

2 x 10-13 g x 1 mol x 6.022 x 1023 molecules

114.21 g 1 mol

= 1 x 109 molecules C6H10S