Wednesday, March 19th: “A” DayThursday, March 20th: “B” Day

AgendaCollect Homework: pg. 21/22 practice

worksheetContinue Section 7.1: “Avogadro’s Number &

Molar Conversions”Homework :

“Grams/Moles Calculations” worksheetConcept Review: “Avogadro’s Number &

Molar Conversions”: #6-10

Continue 7.1: “Avogadro’s Number and Molar Conversions”

6.022 X 1023 atoms 1 mole

Molar Mass Links Moles to Grams

Molar mass: the mass in grams of one mole of an element or compound.

Molar mass is numerically equal to the atomic mass of elements (Fe, Al, etc.) and the formula mass of compounds (H2O, CH4, etc. ) and diatomic elements. (O2, F2, etc.)

** The units for molar mass are **

grams/mole

Molar Mass Example #1What is the molar mass of water, H2O?

1.Use the period table to find the atomic mass of each element in the compound.

2.Add them up to get the molar mass of the compound:

H = 2(1.01 g/mol)O = 16.00 g/mol

Molar mass of water = 18.02 g/mol

Molar Mass Example #2What is the molar mass of sulfuric acid, H2SO4?

H = 2(1.01 g/mol)S = 32.07 g/mol

O = 4(16.00 g/mol)

Molar mass of sulfuric acid = 98.09 g/mol

Molar Mass Example #3 What is the molar mass of sodium carbonate,

Na2CO3?

Na = 2(22.99 g/mol)C = 12.01 g/mol

O = 3(16.00 g/mol)

Molar mass of sodium carbonate =

105.99 g/mol

So, why do we need to know the molar mass…

Molar mass can be used as a conversion factor to change from mass (in grams) to amount (in

moles) !!

Changing Moles Mass (in grams)

Determine the mass in grams of 3.50 mol of copper, Cu.

1.Start with what you know: 3.50 mol Cu2.Use the molar mass of Cu as the conversion

factor to change mol Cu grams Cu.

3.50 mol Cu X 63.55 g Cu = 222 g Cu 1 mol Cu (3 sig figs)

Changing Moles Mass (in grams)Example #1

What is the mass in grams of 10.5 mol ammonia, NH3?

1.Start with what you know: 10.5 mol NH3

2.Use the periodic table to calculate the molar mass of NH3 to use as a conversion factor:

N: 14.01 g/molH: 3(1.01) g/mol

17.04 g/mol

Changing Moles Mass (in grams)Example #1, cont.

3. Multiply by the conversion factor to change mol NH3 grams NH3.

10.5 mol NH3 X 17.04 g NH3 = 179 g NH3

1 mol NH3 (3 sig figs)

Changing Moles Mass (in grams)Example #2

How many grams are in 1.26 moles of aluminum hydroxide, Al(OH)3?

1.Start with what you know: 1.26 moles Al(OH)3

2.Use the periodic table to calculate the molar mass of Al(OH)3 to use as a conversion factor:

Al: 26.98 g/molO: 3(16.00 g/mol)H: 3(1.01 g/mol)

78.01 g/mol

Changing Moles Mass (in grams)Example #2, cont.

3. Multiply by the conversion factor to change mol Al(OH)3 grams Al(OH)3.

1.26 mol Al(OH)3 X 78.01 g Al(OH)3 =

1 mole Al(OH)3

98.3 g Al(OH)3

(3 sig figs)

Do you think we can go the other way and change mass in grams to

amount in moles..

Of course we can, and that’s what we’re going to do…

Changing Mass (in grams) Moles

How many moles of helium are contained in 0.255 g He?1.Start with what you know: 0.255 g He2.Use the molar mass of He as the conversion factor to

change mass He mol He

0.255 g He X 1 mole He = .0638 mol He 4.00 g He (3 sig figs)

Changing Mass (in grams) MolesExample #1

How many moles are there in 68 g of copper (II) hydroxide, Cu(OH)2?

1.Start with what you know: 68 g Cu(OH)2

2.Use the periodic table to calculate the molar mass of Cu(OH)2 to use as a conversion factor:

Cu: 63.55 g/molO: 2(16.00 g/mol)H: 2(1.01 g/mol)

97.57 g/mol

Changing Mass (in grams) MolesExample #1, cont.

3. Multiply by the conversion factor to change mol Cu(OH)2 grams Cu(OH)2.

68 g Cu(OH)2 X 1 mol Cu(OH)2 =

97.57 g Cu(OH)2

.70 mol Cu(OH)2

(2 sig figs)

Changing Mass (in grams) MolesExample #2

How many moles are there in 1,200 g of ammonia, NH3?

1.Start with what you know: 1,200 g NH3

2.Use the periodic table to calculate the molar mass of NH3 to use as a conversion factor:

N: 14.01 g/molH: 3(1.01 g/mol)

17.04 g/mol

Changing Mass (in grams) MolesExample #2, cont.

3. Multiply by the conversion factor to change grams NH3 moles NH3.

1,200 g NH3 X 1 mol NH3 =

17.04 g NH3

70. mol NH3

(2 sig figs)

Homework

You may work with a partner to complete the “Grams/Moles Calculations” worksheet.

You must show all work to get credit!