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Chapter 6 Chemical Reactions and Quantities

6.5Molar Mass

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The molar mass is the mass of one mole

of a substance the atomic mass of an

element expressed in grams

Molar Mass

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Give the molar mass to the nearest 0.1 g.

A. K = ________

B. Sn = ________

Learning Check

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Give the molar mass to the nearest 0.1 g.

A. K = 39.1 g

B. Sn = 118.7 g

Solution

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Molar Mass of CaCl2

For a compound, the molar mass is the sum of the molar masses of the elements in the formula. We calculate the molar mass of CaCl2 to the nearest 0.1 g as follows.

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 1 111.1 g

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Molar Mass of K3PO4

Determine the molar mass of K3PO4 to 0.1 g.

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 1 212.3 g

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One-Mole Quantities

32.1 g 55.9 g 58.5 g 294.2 g 342.3 g

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Guide to Calculating Molar Mass

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Calculate the molar mass for Al(OH)3 to the nearest 0.1g.

1) 44.0 g/mole

2) 75.0 g/mole

3) 78.0 g/mole

Learning Check

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STEP 1 Obtain the molar mass of each element. Al 27.0 g/mole O 16.0 g/mole

H 1.0 g/moleSTEP 2 Multiply each by the subscript in the formula.

1 mole Al x 27.0 g Al = 27.0 g 1 mole Al

3 moles O x 16.0 g O = 48.0 g 1 mole O

3 moles H x 1.0 g H = 3.0 g 1 mole H STEP 3 Total: 1 mole of Al(OH)3 = 78.0 g (3)

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) 40.0 g/mole

2) 262 g/mole

3) 309 g/mole

Learning Check

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3) 309 g/mole

STEP 1 Obtain the molar mass of each element. C 12.0 g/mole H 1.0 g/mole

F 19.0 g/mole N 14.0 g/mole O 16.0 g/mole

STEP 2 Multiply each by the subscript in the formula. 17 moles C x 12.0 g C = 204 g

1 mole C 18 moles H x 1.0 g H = 18 g

1 mole H

Solution

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STEP 2 (continued) 3 moles F x 19.0 g F = 57.0 g

1 mole F 1 mole N x 14.0 g N = 14.0 g

1 mole N

1 mole O x 16.0 g O = 16.0 g 1 mole O

STEP 3 Total: 1 mole of C17H18F3NO = 309 g

Solution (continued)

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Methane (CH4), known as natural gas, is used in gas stoves and gas heaters.

1 mole of CH4 = 16.0 g of CH4

The molar mass of methane can be written as conversion factors.

16.0 g CH4 and 1 mole CH4

1 mole CH4 16.0 g CH4

Conversion Factors from Molar Mass

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Acetic acid, C2H4O2, gives the sour taste to vinegar.

Write an equality and two molar mass conversion factors for acetic acid.

Learning Check

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Acetic acid, C2H4O2, gives the sour taste to vinegar.

Write an equality and two molar mass conversion factors for acetic acid.

Equality:

1 mole of acetic acid = 60.0 g of acetic acid

Molar mass conversion 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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Guide to Calculations Using Molar Mass

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Aluminum is often used for the structure of lightweight bicycle frames. How many grams of Al are in 3.00 moles of Al?

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

mole factor for Al

Calculating Grams from Moles

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The artificial sweetener aspartame (NutraSweet), C14H18N2O5 , is used to sweeten diet foods, coffee, and soft drinks. How many moles of aspartame are present in 225 g of aspartame?

Learning Check

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STEP 1 Given: 225 g of aspartame, C14H18N2O5

STEP 2 Plan: g of aspartame moles of aspartame

STEP 3 Determine molar mass of aspartame:

14(12.0) + 18(1.0) + 2(14.0) + 5(16.0)

= 294 g/mole

STEP 4 Set up problem to calculate the moles of aspartame:

225 g aspartame x 1 mole aspartame

294 g aspartame mole factor (inverted)

= 0.765 mole of aspartame

Solution

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Converting Between Mass, Moles, and Particles

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