Chemistry Final Review 5: Formulas and Moles

Chemistry Final Review 5: Formulas and Moles

By William Lovering

Chickens and Elephants

• Which has more, a dozen chickens or a dozen elephants?

• Which weighs more, a dozen chickens or a dozen elephants?

• Which has more atoms, a dozen C or a dozen H? Which weighs more?

Comparisons

• Which has more animals - a mole of chickens or a mole of elephants?

• Which weighs more, a mole of C or a mole of H?

• How do we find the mass of a mole of any element or compound?

​Facts on The Mole

• The mole can either be a mass or a number of particles

• ​Abbreviation is 'mol'

• ​Commonly used term is molar mass = mass of one mole of a substance

  • has the units of g/mol or g.mol^-1

• We can easily convert between mass, moles, and particles

• We can also easily calcul​ate mass, molar mass, or moles (as long as we have two of these numbers)

​

Molar Mass

mass of 1 mole

determined from the periodic table

keep sig figs as they are on the table (usually 2 decimal places)

Molar Mass

• molar mass = 1 mole

• CO₂

• C=12 O=16x2

• 44 g/mol

Molar Mass

1 mole of atoms of any element will have a mass equal to its relative Atomic Mass

• Its shown in grams​

​Once Again

​1 Mol = 23 grams

2 Mol =

3 Mol =

​Examples

How many grams are in 2 mol of Carbon (C)?

1. Look for the element

2. Look at the atomic mass

3. multiply the mol they are asking for​

Formula Weight

​Its the sum of all the atomic weights in a formula.

to find the formula weight of a compound:

1. fin the number of atoms of each element present in the compound

2. multiply the number of atoms by their respective weight

3. add the atomic weights of all the elements present.

Example: H₂O

1. H₂O contains 2 atoms of Hydrogen and 1 atom of Oxygen​

   ​

2. Hydrogen: 2 x 1.0080 = 2.016

Oxygen: 1 x 16 = 16

​

3. 2.016 + 16 = 18.016

​Example

Determine the gram formula mass of each compound: KMnO₄

(this is asking for the mass of one mol of your compound)

​

K = 1 x 39.098 = 39.098

Mn = 1 x 54.938 = 54.938

O = ​4 x 16 = 64

= 158.036​ g/mol

​Number of grams in 1 mole

Number of grams in 1 mole

Equal to the numerical value of the atomic mass

1 mole of C atoms = 12.0 g

1 mole of Mg atoms = 24.3 g

1 mole of Cu atoms = 63.5 g

Percent Composition

The percent of a specific element in a chemical formula

How to find percent composition

• Step 1: Find the total molar mass of the compound

• Step 2: Place the entire mass of the specific element you want to find in the numerator and the total molar mass you found in step one in the denominator

• Divide the numbers and multiply by 100

Multiple Choice

What is the percent by mass of fluorine in CaF₂ (gram-formula mass = 78 g/mol)?

• 24%
• 49%
• 51%
• 65%

Multiple Choice

What is the percent by mass of oxygen in MgO?

• 20%
• 40%
• 50%
• 60%

Multiple Choice

Which would have more atoms?

• 1 mole of Li
• 1 mole of Au
• 1 mole of Si
• None, all are equal

Multiple Choice

How many Carbon Atoms are in 4 moles of Carbon?

• 6.02 x 10²³

• 1.33 x 10²³

• 4.52

• 2.408 x 10²⁴

Multiple Choice

The number 6.02 x 10²³ is called...

• Molar number

• Bohr's number
• Scientist's number

• Avogadro's number

Multiple Choice

You try, just to make sure you're ready to move on to this level of material:

If you had 3 moles of Sodium, how many grams of sodium would you have?

• 68.97 g Na

• 7.663 g Na

• 0.1305 g Na

• 1.807E24 atoms Na

Multiple Choice

For the compound Fe₂O₃, if we had 4 moles of Iron (Fe), how many moles of oxygen would we have?

• 6 moles O

• 3 moles O

• 4 moles O

• 9 moles O

Multiple Choice

What is the mass of 4.6 x 10¹⁸ molecules of boric acid, H₃BO₃?

• 4.9 x 10^-6

• 4.7 x 10^-4

• 3.9 x 10^-17

• 7.7 x 10^-2

Multiple Choice

The molar mass of an element is equal to its...

• Atomic mass
• Atomic number
• Oxidation number
• Valence electrons

Multiple Choice

What is the molar mass of table salt (NaCl)?

• 116.886 g/mol
• 35.453 g/mol
• 22.990 g/mol
• 58.443 g/mol

Multiple Choice

Calculate the molar mass of NH₄NO₂

• 50 g/mol

• 60 g/mol

• 64 g/mol

• 78 g/mol