Bell Ringer What is a Mole? What is the mass of a NaCl molecule?

Chapter 7.1b Molar Mass

Common counting units: shoes by the pair (pair = 2) eggs by the dozen (dozen = 12) paper by the ream (ream = 500 sheets).

Counting With Moles Chemical reactions usually involve very large numbers of tiny particles We use a counting unit called the mole to measure amounts of a substance. Mole (mol) = amount of any substance that contains approximately 6.02 × 1023 particles. This amount is known as Avogadro’s number. Ex: 1 mole of iron = 6.02 × 1023 atoms of iron.

Molar Mass: mass of one mole of a substance same as atomic mass, in grams. Examples: Carbon: Atomic mass = 12.0 amu Molar mass = 12.0 grams. Sulfur: Atomic mass = 32.1 amu Molar mass = 32.1 grams.

Counting With Moles Molar Mass of a compound = sum of atomic masses of all its atoms, expressed in grams. Tells us the total grams in one mole of the compound Example: Carbon dioxide (CO2) 1 carbon atom = 12.0 amu 2 oxygen atoms = (2 × 16.0 amu) = 32.0 amu CO2 atomic mass = (12.0 + 32.0) = 44.0 amu CO2 molar mass = 44.0 grams 44.0g CO2 in 1 mol CO2

Mole-Mass Conversions Knowing the molar mass of a substance lets you easily convert between moles and mass “converting” means you can calculate one from the other Example: Molar mass of CO2 is 44.0 grams 1.0 mole of CO2 = 44.0 grams Conversion factors  OR

Converting Example: Chocolate Cake Recipe Recipe calls for 4 eggs per cake and you plan to make 3 cakes. Eggs can only be bought by the dozen (i.e. groups of 12). How may dozen eggs do you need to buy? Conversion factors: 4 eggs 1 cake 1 cake 4 eggs 12 eggs 1 doz. eggs 1 doz. eggs 12 eggs Calc: 3 cakes x 4 eggs x 1 doz eggs = 1 doz eggs 1 cake 12 eggs

Counting With Moles You have 55.0 grams of CO2. How many moles? To calculate how many moles of CO2 you have, multiply the mass by a conversion factor. You can check your answer.

Chemical Calculations In chemical reactions, the mass of a reactant or product can be calculated. Use a balanced chemical equation and the molar masses of the reactants and products.

Chemical Calculations In a balanced chemical equation, the number of atoms of each element on the left equals the number of atoms of each element on the right.

Chemical Calculations The Coefficient tells us the # of moles of each reactant and product involved the reaction

Chemical Calculations Molar Mass = the # of grams per mole of each reactant and product

Chemical Calculations Mass (of a reactant or product) = the # of moles in the balanced equation (Coefficient) x the molar mass.

Chemical Calculations - Stoichiometry Converting Mass to Moles To calculate how much oxygen is required to make 144 grams of water, begin with a balanced chemical equation for the reaction. 2H2 + O2  2H2O Determine how many moles of water you are trying to make. Convert the given mass (144g) of water into moles.

Chemical Calculations Using Mole Ratios Write the conversion factors, or mole ratios. 2H2 + O2  2H2O Calculate how many moles of oxygen are required to produce eight moles of water.

Chemical Calculations Converting Moles to Mass Convert moles of O2 to grams of O2 by using the molar mass of O2 as a conversion factor. To produce 144 grams of H2O, you must supply 128 grams of O2.

Assessment Questions Which of the following is a balanced chemical equation for the reaction of aluminum and ammonium perchlorate? Al + NH4ClO4 —› Al2O3 + NH4Cl 4Al + 3NH4ClO4 —› 4Al2O3 + 3NH4Cl 8Al + NH4ClO4 —› 4Al2O3 + NH4Cl 8Al + 3NH4ClO4 —› 4Al2O3 + 3NH4Cl

Assessment Questions Which of the following is a balanced chemical equation for the reaction of aluminum and ammonium perchlorate? Al + NH4ClO4 —› Al2O3 + NH4Cl 4Al + 3NH4ClO4 —› 4Al2O3 + 3NH4Cl 8Al + NH4ClO4 —› 4Al2O3 + NH4Cl 8Al + 3NH4ClO4 —› 4Al2O3 + 3NH4Cl ANS: D

Assessment Questions How many moles of NaCl (molar mass = 58.5) are there in a 5.85-gram sample? 58.5 mol 5.85 mol 0.10 mol 0.28 mol

Assessment Questions How many moles of NaCl (molar mass = 58.5) are there in a 5.85-gram sample? 58.5 mol 5.85 mol 0.10 mol 0.28 mol ANS: C

Assessment Questions In the reaction shown below, how many grams of hydrogen are produced when 10 moles of potassium react with water? 2K + 2H2O —› 2KOH + H2 2.5 g 5 g 10 g 20 g

Assessment Questions In the reaction shown below, how many grams of hydrogen are produced when 10 moles of potassium react with water? 2K + 2H2O —› 2KOH + H2 2.5 g 5 g 10 g 20 g ANS: C

Assessment Questions In the chemical equation below, the reactants are carbon dioxide and water. CH4 + 2O2 —› CO2 +2H2O True <or> False? ANS: False, products