The Mole Introduction to Chemistry

The Mole Atoms and molecules are too small to count out individually Avogadro’s Number = 6.02 x particles / mole One mole is equal to exactly 6.02 x particles of any given compound or element 6.02 x particles of “X”= 1 mole of “X”

The Mole 6.02 x particles / mole means that any of the following are true: 6.02 x atoms / mole 6.02 x molecules / mole 6.02 x ions / mole 6.02 x formula units / mole

The Mole Example: How many molecules of water are there in 3.0 moles of water? 1.8 x molecules of water

The Mole Example: How many atoms are there in 5.21 moles of carbon? 3.14 x atoms of C

The Mole Example: If we have 9.28 x formula units of BaCl 2, how many moles of barium chloride do we have? 1.54 x moles of BaCl 2

The Mole Example: How many moles of magnesium are in 4.21 x atoms of magnesium? moles of magnesium

The Mole Two-step problems: Example: How many atoms of oxygen are in 3.9 moles of carbon dioxide? 4.7 x atoms of O

The Mole Example How many hydroxide ions are there in 13.2 moles of Mn(OH) 3 ? 2.38 x hydroxide ions

The Mole Example How many sulfate ions in 7.25 moles of Al 2 (SO 4 ) 3 ? 1.31 x sulfate ions

Converting from Grams to Moles Using Molar Mass Molar Mass the mass of one mole of a given compound or element Elements the molar mass of an element is simply it’s atomic mass with grams/mole as units Example: Find the molar mass of Nitrogen 14.0 g/mol

Molar Mass Converting from Grams to Moles Example: How many moles of sodium are in grams of sodium? 2.45 mol Na

Molar Mass Compounds use the “SSMT” method to find the molar mass Examples: H 2 O Na 2 CO 3 Pb(NO 3 ) g/mol g/mol 18.0 g/mol

Using Molar Mass We can use molar mass to find the mass of an amount of substance. Example: What is the mass (in grams) of 1.50 mole of potassium sulfate? 261 grams of K 2 SO 4

Using Molar Mass Example: What is the mass of 2.90 moles of Cu(NO 3 ) 2 ? 544 grams of copper nitrate

Using Molar Mass We can use molar mass to do the following conversions: Particles → moles → grams Grams → moles → particles Note: you can never go straight from particles to grams! You must always convert to moles first (using molar mass)

Using Molar Mass Example: If you have 39 grams of water in a beaker, how many molecules of water are present? 1.3 x molecules of water

Using Molar Mass Example: If you have 4.21 x atoms of gold, how many grams of gold do you have? 138 grams of gold

Molar Volume Molar volume – the volume of one mole of a gas at standard temperature and pressure (22.4 L/mol) Standard temperature and pressure (STP) temp. = 0ºC, pressure = 1 atmosphere

Molar Volume Example How many moles of gas are present in 12.5 liters of gas at STP? moles of gas

Molar Volume Example How many moles of gas are present in 82 liters of gas at STP? 3.7 moles of gas

Molar Volume Example How many liters of gas are present in 2.1 moles of gas at STP? 47 liters of gas

Percent Composition the percentage of mass that each element contributes to the total mass of the compound % Comp. = Molar mass of element x 100 Molar mass of entire compound

Percent Composition Example: Find the percent composition for each element in Al 2 (SO 4 ) 3 % Al = 15.8 % % S = 28.1 % % O = 56.1 %

Percent Composition Example: Find the percent composition for each element in Mg(OH) 2 % Mg = 41.7 % % O = 54.9 % % H = 3.4 %

Empirical Formula Molar mass can be used to determine a compound’s empirical formula Steps: 1. Convert each element’s mass to moles 2. Divide by the smallest number of moles 3. Round to the nearest whole number and use as subscripts

Empirical Formula Example: What is the empirical formula for a compound that has 1.80 g of carbon and 4.98 g of oxygen? CO 2

Empirical Formula Example: What is the empirical formula for a compound that has g of carbon, g hydrogen, and g of nitrogen? CH 5 N

Empirical Formula Using Percent Composition Steps 1. Assume a 100 g sample 2. Convert from grams to moles 3. Divide by the smallest number of moles 4. Round to the nearest whole number and use as subscripts

Empirical Formula Example: What is the empirical formula for a compound that has a percent composition of 40.0% carbon, 6.71% hydrogen, and 53.3% oxygen? CH 2 O

Empirical Formula Example: If the empirical formula for a compound is CH 2 Cl and the molar mass for the molecular formula is g/mol, what is the molecular formula? molecular = C 2 H 4 Cl 2

Empirical/Molecular Formula Example: If empirical formula for a compound is CH 2 O and the molar mass of the molecular formula is g/mol, what is the molecular formula? molecular = C 2 H 4 O 2

SOL covered during lesson CH 4 a, b CH 3 c

Extra Slides

Empirical/Molecular Formula Example: Caffeine has a percent composition of 49.48% carbon, 5.15% hydrogen, 28.87% nitrogen, and 16.49% oxygen. If the molar mass of the compound is g/mol, what is the molecular formula of caffeine? C 8 H 10 N 4 O 2

Empirical Formula Example: What is the empirical formula for a compound that has a percent composition of 71.65% chlorine, 24.27% carbon, and 4.07% hydrogen? If the molar mass for the molecule is g/mol, what is the molecular formula? empirical = CH 2 Cl molecular = C 2 H 4 Cl 2

Empirical/Molecular Formula Example: What is the empirical formula for a compound that has a percent composition of 40.0% carbon, 6.71% hydrogen, and 53.3% oxygen? If the molar mass of the compound is g/mol, what is the molecular formula? empirical = CH 2 O molecular = C 2 H 4 O 2