Mass and the Mole

1
Mass and the Mole

• Chemistry
• Section 11.2

2
Objectives

• Relate the mass of an atom to the mass of a mole
  of atoms.
• Calculate the number of moles in a given mass of
  an element and the mass of a given number of
  moles of an element.
• Calculate the number of moles of an element when
  given the number of atoms of the element.
• Calculate the number of atoms of an element when
  given the number of moles of the element.

3
The Mass of the Mole

• Do a dozen limes have the same mass as a dozen
  eggs?
• No.
• The same is true for moles.
• One mol of Fe does not have the same mass as one
  mol of Ag.

4
Molar Mass

• This is IMPORTANT!
• Mass in grams of 1 mole of any pure substance is
  termed the molar mass.
• Molar mass of any element equals its atomic mass
• Units are g/mol

5
Molar Mass

• For Example
• Manganese
• What is Manganeses molar mass?
• 54.94 amu OR g/mol
• What does one mole of Manganese weigh?
• 54.94 g
• How many atoms of Manganese are in one mol?
• 6.022 x 1023

6
Molar Mass

• We can use the molar mass of every element as a
  conversion factor.
• Example
• of mol x of grams mass
• 1 mol

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Mole to Mass Conversions

• Chromium (Cr) is a transition element.
• Calculate the mass in grams of 0.0450 moles of
  Cr.
• Always start with ?
• What is given in the problem!
• 0.0450 mol Cr

8
Mole to Mass Conversions

• Molar mass is the conversion factor
• 52.00 g/1 mol Cr
• Put what you want to end up with on the top of
  the conversion factor.

9
Mole to Mass Conversions

• 0.0450 mol Cr x 52 g 2.34 g Cr
• 1 mol

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Practice Problems

• Pg. 316
• 12

11
Mass to Atoms Conversions

• Lets take the conversion factor further.
• 2 step conversion factor problems

Representative Particles
Mass
Moles
12
Mass to Atoms Conversions

• If given the mass of an element, you can find the
  number of atoms in that mass.
• Example How many atoms of gold are in a pure
  gold nugget having a mass of 25.0 g?

13
Mass to Atoms Conversions

• Always start with?
• What is given in the problem.
• 25.0 g Au
• Mass ? Moles ? Particles

14
Mass ? Moles

• Use molar mass of Au conversion factor
• 196.97 g
• 1 mol Au

15
Moles ? Particles

• Avogadros conversion
• 6.022 x 1023 atoms
• 1 mol Au

16
Mass?Moles? Atoms
1st conversion 2nd conversion

• 25.0 g Au x 1 mol Au x 6.022 x 1023 atoms
• 196.97g Au 1 mol Au
• 7.65 x 1022 atoms Au

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Practice Problems

• Pg 318
• 14

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Homework

• Molar mass worksheet