1 Chapter 5 Chemical Quantities and Reactions 5.2 Molar Mass Copyright © 2009 by Pearson Education, Inc.

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Chapter 5 Chemical Quantities and Reactions

5.2Molar Mass

Copyright © 2009 by Pearson Education, Inc.

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Molar Mass

The molar mass

• is the mass of 1 mole of an element or compound.

• is the atomic or molecular mass expressed in grams.

Copyright © 2009 by Pearson Education, Inc.

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Molar Mass from Periodic Table

Molar mass is the atomic mass expressed in grams.

1 mole of Ag 1 mole of C 1 mole of S = 107.9 g = 12.01 g = 32.07 g

Copyright © 2009 by Pearson Education, Inc.

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Give the molar mass for each (to the tenths decimal

place).

A. 1 mole of K atoms = ________

B. 1 mole of Sn atoms = ________

Learning Check

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Give the molar mass for each (to the tenths decimal

place).

A. 1 mole of K atoms = 39.1 g

B. 1 mole of Sn atoms = 118.7 g

Solution

Guide to Calculation Molar Mass of a Compound

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Molar Mass of a Compound

The molar mass of a compound is the sum of the molar masses of the elements in the formula.

Example: Calculate the molar mass of CaCl2.

Element Number of Moles

Atomic Mass Total Mass

Ca 1 40.1 g/mole 40.1 g

Cl 2 35.5 g/mole 71.0 g

CaCl2 111.1 g

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Molar Mass of K3PO4

Calculate the molar mass of K3PO4.

Element Number of Moles

Atomic Mass Total Mass in K3PO4

K 3 39.1 g/mole 117.3 g

P 1 31.0 g/mole 31.0 g

O 4 16.0 g/mole 64.0 g

K3PO4 212.3 g

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Some 1-mole Quantities

32.1 g 55.9 g 58.5 g 294.2 g 342.2 g

One-Mole Quantities

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What is the molar mass of each of the following?

A. K2O

B. Al(OH)3

Learning Check

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A. K2O 94.2 g/mole

2 moles of K (39.1 g/mole) + 1 mole of O (16.0 g/mole) 78.2 g + 16.0 g = 94.2 g

B. Al(OH)3 78.0 g/mole

1 mole of Al (27.0 g/mole) + 3 moles of O (16.0 g/mole) + 3 moles of H (1.01 g/mole) 27.0 g + 48.0 g + 3.03 g = 78.0 g

Solution

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Prozac, C17H18F3NO, is an antidepressant that inhibits the uptake of serotonin by the brain. What is the molar mass of Prozac?

1) 40.1 g/mole2) 262 g/mole 3) 309 g/mole

Learning Check

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Prozac, C17H18F3NO, is an antidepressant that inhibits the uptake of serotonin by the brain. What is the molar mass of Prozac?

3) 309 g/mole

17 C (12.0) + 18 H (1.01) + 3 F (19.0) + 1 N (14.0)

+ 1 O (16.0) =

204 g C + 18.2 g H + 57.0 g F + 14.0 g N + 16.0 g

O = 309 g/mole

Solution

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Molar mass conversion factors • are fractions (ratios) written from the molar mass.• relate grams and moles of an element or compound.• for methane, CH4, used in gas stoves and gas heaters

is1 mole of CH4 = 16.0 g (molar mass

equality)

Conversion factors:

16.0 g CH4 and 1 mole CH4

1 mole CH4 16.0 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

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Calculate molar mass for acetic acid C2H4O2:

24.0 g C + 4.04 g H + 32.0 g O = 60.0 g/mole C2H4O2

1 mole of acetic acid = 60.0 g acetic acid

Molar mass factors:1 mole acetic acid and 60.0 g acetic acid 60.0 g acetic acid 1 mole 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 often used to build lightweight bicycleframes. How many grams of Al are in 3.00 mole of Al?

Molar mass equality: 1 mole of Al = 27.0 g of Al

Setup with molar mass as a factor:

3.00 mole Al x 27.0 g Al = 81.0 g of Al1 mole Al

molar mass factor for Al

Moles to Grams

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Learning Check

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

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Calculate the molar mass of C6H10S.

(6 x 12.0) + (10 x 1.01) + (1 x 32.1) = 114.2 g/mole

Set up the calculation using a mole factor.

225 g C6H10S x 1 mole C6H10S

114.2 g C6H10S

molar mass factor (inverted)

= 1.97 moles of C6H10S

Solution

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Grams, Moles, and Particles

A molar mass factor and Avogadro’s number convert…

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Learning Check

How many H2O molecules are in 24.0 g of H2O?

1) 4.52 x 1023 H2O molecules

2) 1.44 x 1025 H2O molecules

3) 8.03 x 1023 H2O molecules

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Solution

How many H2O molecules are in 24.0 g of H2O?

3) 8.02 x 1023

24.0 g H2O x 1 mole H2O x 6.02 x 1023 H2O molecules 18.0 g H2O 1 mole H2O

= 8.03 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 1 liter of air, how many molecules of C6H10S are present?

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Solution

If the odor of C6H10S can be detected from 2 x 10-13 g in 1 liter of air, how many molecules of C6H10S are present?

2 x 10-13 g x 1 mole x 6.02 x 1023 molecules 114.2 g 1 mole

= 1 x 109 molecules of C6H10S are present