Grouping!

• When with many items in a large set need to be counted, it is often useful to work with groups of items rather than individual items.

Item Quantity Amount

Gloves Pair 2

Cans of pop

Six-pack 6

Eggs Dozen 12

Pens Gross (12 dozen)

144

Paper Ream 500

• Chemists use a quantity to group atoms and molecules that is much larger than a dozen or a ream - THE MOLE (mol)

• The mole is defined as the amount of substance that contains as many entities (atoms, molecules, or units) as exactly 12 g of Carbon.

• One mole of a substance contains • 6.022 141 99 x 1023 particles

• This number is Avogadro’s constant and for the purposes of this class we will just know it as

• 6.022 x 1023 particles.

• The mass on the periodic table is called the molar mass.

• 55.847 g of iron is one mole.

• One mole of iron has 6.022 x 1023 atoms.

• So 55.847 g of iron has 6.022 x 1023 atoms.

Mole Conversions

• The Mole formula! Be able to rearrange it to solve for each of the variables!

n = m MM

Number of moles (mol)

mass (g)

Molar mass (g/mol)

Mole to Mass Example

• 2.001 mol KCl is how many grams?• First you need the molar mass of KCl from the

periodic table.• 39.0983 + 35.4527 = 74.5510 g/mol

• The you need to do the conversion.

• 2.001 mol x 74.5510 g = 149.2 g1 mol

Another Mole to Mass Example

• How many grams is in 0.42 mol of Mg(OH)2?

• Molar mass of Mg(OH)2 is• 24.3050 + 2(15.9994) + 2(1.00794) = 58.3197 g/mol

• 0.42 mol x58.3197 g = 24 g1 mol

Mass to Mole Example

• You are given 157 g NaOH. How many moles is this?

• To solve this, you need the molar mass from the periodic table.

• 22.989768 + 15.9994 + 1.00794 = 39.9971 g/mol

• 157 g NaOH x 1 mol = 3.93 mol 39.9971g

Another Mass to Mole example

• How many moles are in 1526 g K2CO3?

• Molar mass of K2CO3 is • 2(39.0983) + 12.011 + 3(15.9994) = 138.197 g/mol

• 1526 g x 1 mol = 11.04 mol138.197 g

Moles to Particles Example

• How many particles are in 10.8 mol FeO?• The only thing you need for this conversion is

Avogadro’s constant (6.022 x 1023)

• 10.8 mol FeO x 6.022 x 1023 particles1 mol

• 6.50 x 1024 particles

Another Mole to Particle Example

• How many atoms are in 17.6 mol of CaCl2?

• Avogadro’s constant is 6.022 x 1023

• 17.6 mol CaCl2 x 6.022 x 1023 atoms

1 mol• 1.06 x 1025 atoms

Moles to Volume at STP Example

• Volume at standard temperature (25oC) and pressure (1 atm) is a constant of 22.4 L.

• How many L are in 25.5 mol Al2O3?

• All you need to do the conversion is the constant of 22.4 L.

• 25.5 mol x 22.4 L = 571 L1 mol

Volume to Mole Example at STP

• How many moles in 6.85 L of Cl2 gas?

• All you need is the constant.

• 6.85 L x 1 mol = 0.306 mol22.4 L

• When you calculate and use the molar mass of a compound, you are making an important assumption.

• Say you are working with water, you are assuming that every sample of water contains two hydrogen and one oxygen and the mass ratio is 2(1.00794) to 1(15.9994).

• At this point, now that we have the periodic table, this two-to-one ratio seems obvious. But when scientists first discovered that compounds contained elements in fixed mass proportions, they did not have the periodic table.

• In fact, the discovery of fixed mass proportions was an important step toward the development of atomic theory.

FYI• There are an infinite number of

significant digits in a mole, so don’t assume it to have only 1!

• You MUST convert ALL quantities into standard units (L, g, etc) to use in all formulas

• Molar masses for molecules are calculated based on their FORMULA

• For full marks you MUST SHOW ME ALL STEPS, FORMULAS, INCLUDE FINAL STATEMENTS and correct SIGNIFICANT DIGITS

• My advise… carry your units through all calculations so that you can be sure your answer is correct when they cancel out properly