UNIT 4: THE MOLE & AVOGADRO

Transcription

1 UNIT 4: THE MOLE & AVOGADRO SC.912.P.8.9 Apply the mole concept & Law of Conservation of Mass to calculate the quantities of chemicals SC.912.P.8.7 Interpret formula representations of molecules & compounds in terms of composition & structure

2 VOCABULARY (ADD TO YOUR REFERENCE FOLDER DICTIONARY!) Molecular formula: a formula that gives the number of atoms of each element present in a molecule. (e.g. H2O, NaCl, C2H6) Avogadro s Number: A constant value; The number of formula units (atoms, molecules, particles) in one mole of that substance = 1 mole (mol) Atomic Mass Unit (amu): the mass of an atom; found on the Periodic Table. Molar Mass: The mass (in grams) of 1 mole of a substance; found on the Periodic Table. Significant Figure Rules: -Add/Subtract: The final calculated answer should have the least # of decimal places -Multiply/Divide: The final calculated answer should have the least # of overall digits

3 Q: WHAT IS A MOLE? A: A CHEMIST S DOZEN Think of the following How many is 1 dozen? How many is 1 pair? In chemistry, we use the same idea! How many is 1 mole? 1 mole = 6.02 x formula units Atoms/molecules/particles are too small to individually count for practical purposes, so we use the term mole to count atoms/molecules/particles more conveniently Imagine if you had cartons of eggs stacked to the ceiling what is the easiest/fastest way to count how many eggs there are? Count 1 egg, 2 eggs, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14 or count by the dozen? 1 dozen, 2 dozen, 3 dozen

4 Q: WHAT IS A MOLE? A: A CHEMIST S DOZEN 1 mole of ANYTHING ALWAYS equals 6.02 x of those things 1 mole of paperclips = 6.02 x paperclips 1 mole of hydrogen atoms = 6.02 x hydrogen atoms 1 mole of donuts = 6.02 x donuts 1 mole of NaCl = 6.02 x molecules of NaCl 1 mole of Mg(NO 3 ) 2 = 6.02 x molecules of Mg(NO 3 ) 2

5 If you are thinking, How much does 1 mole of hydrogen weigh? the unit would be in grams so 1 mole of H atoms would weigh grams. This is called MOLAR MASS The molar mass of any chemical ALWAYS equals 1 mole of that chemical OKAY SO, HOW DO I KNOW WHEN I HAVE 1 MOLE IF I CAN T COUNT THE INDIVIDUAL ATOMS/MOLECULES/PARTICLES? Use the atomic mass found on the Periodic Table! Notice that there are no units listed after the atomic mass If you are thinking, How much does 1 hydrogen atom weigh? the unit would be a.m.u. (atomic mass unit) so 1 H atom weighs amu. This is called FORMULA MASS

6 CONVERSION FACTORS! Avogadro s Number Converts between: Formula Units Moles Moles Formula Units 6.02 x formula units = 1 mole This conversion NEVER CHANGES! It is a CONSTANT Molar Mass Converts between: Grams Moles Moles Grams Must be calculated using the molecular formula STEPS: 1. Get the molecular formula of the molecule 2. Determine which elements are in the molecule 3. Count how many atoms of each element are in the molecule 4. Find the atomic mass of each element using the Periodic Table 5. Multiply the atomic mass of each element by however many atoms are in the element 6. Add them together! This value, in grams, equals 1 mole of the molecule

7 A NOTE ON COUNTING ATOMS IN A MOLECULAR FORMULA MgCl 2 What elements are in this molecular formula? How many of each? What is the atomic mass of each element, according to the Periodic Table? Multiply the mass by the number of atoms, and add these together to determine the molar mass (in grams), or formula mass (in a.m.u.) Mg: 1 x = Cl: 2 x = g MgCl 2 = 1 mol MgCl 2 (molar mass) or the formula mass of MgCl 2 is a.m.u. Ca(OH) 2 What elements are in this molecular formula? How many of each? Distribute the 2 to what is INSIDE the parenthesis by MULTIPLYING, like in math! What is the atomic mass of each element, according to the Periodic Table? Multiply the mass by the number of atoms, and add these together to determine the molar mass (in grams), or formula mass (in a.m.u.) Ca: 1 x = O: 2 x = H: 2 x = g Ca(OH) 2 = 1 mol Ca(OH) 2 (molar mass) or the formula mass of Ca(OH) 2 is a.m.u.

8 2 TYPES OF FORMULAS MOLECULAR FORMULA A formula that gives the number of atoms of each element present in a molecule. E.g. H 2 O, NaCl, C 2 H 6 EMPIRICAL FORMULA A formula that gives the simplest ratio of elements present in a molecule. Instead of C 2 H 6, the empirical formula is CH 3 Instead of C 6 H 12 O 6, the empirical formula is CH 2 O NOTE: If the molecular formula is already in a reduced form, then the empirical formula is the SAME as the molecular formula!

9 2 TYPES OF FORMULAS Search YouTube for: Writing Empirical Formula Practice Problems by Tyler DeWitt

10 PERCENT COMPOSITION The percent composition of a component (element) in a compound is the percent of the total mass of the compound that is due to that component (element). Percent Composition = Mass due to a specific element Total molar mass of the compound 100 STEPS: 1. Calculate the molar mass of the compound. 2. Determine how much of that mass is due to a particular component (element) to put more simply, calculate the molar mass of the particular component (element) in question multiplied by the number of atoms of this element. 3. Divide the mass of the particular component (from step 2) by the molar mass of the compound (from step 1), then multiply this quotient by 100.

11 PERCENT COMPOSITION EXAMPLE: Calculate the percent composition of carbon in CO2. Step 1 Calculate molar mass of the compound C: 1 x = O: 2 x = g CO2 = 1 mol CO2 Step 2 Calculate the mass of the molecule DUE TO CARBON 1 x Step 3 Divide step 2 by step 1, then multiply by Percent Composition = Mass due to a specific element Total molar mass of the compound = % This means that % of CO2 is carbon

12 PERCENT COMPOSITION EXAMPLE: Calculate the percent composition of carbon in C 6 H 12 O 6 Step 1 Calculate molar mass of the compound C: 6 x = H: 12 x = O: 6 x = g C 6 H 12 O 6 = 1 mol C 6 H 12 O 6 Step 2 Calculate the mass of the molecule DUE TO carbon 6 x ( g) Step 3 Divide step 2 by step 1, then multiply by = % Percent Composition = Mass due to a specific element Total molar mass of the compound 100 This means that % of the mass of C 6 H 12 O 6 is carbon