Chapter 7 Lesson 1 Chemical Quantities

Counting Particles By Weighing If a person requests 500 quarter inch hexagonal nuts for purchase If a person requests 500 quarter inch hexagonal nuts for purchase How would you count 500 hex nuts? How would you count 500 hex nuts? a) You could count the hex nuts individually b) Or, count the number of hex nuts by weight First, find the average weight by weighing out 10 hex nuts and obtaining the total weight (10 hex nuts weigh 105 g) First, find the average weight by weighing out 10 hex nuts and obtaining the total weight (10 hex nuts weigh 105 g) Average Weight

Counting Particles By Weighing What size (wt.) will contain 500 hex nuts? What size (wt.) will contain 500 hex nuts? Calculate what weight will contain 500 hex nuts Calculate what weight will contain 500 hex nuts So, weigh out 5.25 kg of the hex nuts So, weigh out 5.25 kg of the hex nuts

Using Atomic Mass to Count Atoms Can do the exact same thing with atoms Can do the exact same thing with atoms Too small to conveniently count Too small to conveniently count Atoms of the same element don’t always have exactly the same mass (isotopes) so use an Average Mass Atoms of the same element don’t always have exactly the same mass (isotopes) so use an Average Mass Grams are too large to use to measure an atom so use Atomic Mass Units (amu) Grams are too large to use to measure an atom so use Atomic Mass Units (amu)

Atomic Mass and Formula Mass To calculate the mass of a sample of atoms To calculate the mass of a sample of atoms Each element exists as a mixture of isotopes Each element exists as a mixture of isotopes Use a “weighted average” for the atomic mass Use a “weighted average” for the atomic mass Number on the bottom of each square in the periodic table is the average weight of the element (in amu) Number on the bottom of each square in the periodic table is the average weight of the element (in amu)

Atomic Mass and Formula Mass Atomic masses are determined on a relative scale Atomic masses are determined on a relative scale The standard scale references the carbon-12 isotope = amu The standard scale references the carbon-12 isotope = amu All other atomic masses are determined relative to carbon-12 All other atomic masses are determined relative to carbon-12

Atomic Mass and Formula Mass Using Atomic Mass to Count Atoms Using Atomic Mass to Count Atoms Calculating the number of atoms in a specific mass Calculating the number of atoms in a specific mass If you have a sample of an element, can calculate the number of atoms in that sample If you have a sample of an element, can calculate the number of atoms in that sample From the atomic mass per one atom a conversion factor can be made From the atomic mass per one atom a conversion factor can be made For example: One nitrogen atom has an atomic mass of amu For example: One nitrogen atom has an atomic mass of amu

Calculating The Number of Atoms in a Specific Mass You have a 1.00 g sample of lead. How many atoms of lead are present? You have a 1.00 g sample of lead. How many atoms of lead are present?  1.00 g Pb1 amu 1.66 x g 1 atom Pb amu 

Calculating Mass Example Calculate the mass (in amu) of 1.0 х 10 4 carbon atoms Calculate the mass (in amu) of 1.0 х 10 4 carbon atoms 1) Given : 2) Plan: Convert from atoms to amu 3) CF 4) Set Up Problem

Formula Mass The sum of atomic masses of all atoms in its formula The sum of atomic masses of all atoms in its formula Important role in nearly all chemical calculations Important role in nearly all chemical calculations Can be calculated for compounds and diatomic elements Can be calculated for compounds and diatomic elements

Calculating Formula Mass Calculate the formula mass of calcium chloride Calculate the formula mass of calcium chloride Write the formula from the name given Write the formula from the name given Ca 2+ (from group II) and Cl - (from group VII) Ca 2+ (from group II) and Cl - (from group VII) Formula is CaCl 2 due to charge balance Formula is CaCl 2 due to charge balance Formula mass: Sum of the atomic masses of atoms in the formula (1 Ca atom + 2 Cl atoms) Formula mass: Sum of the atomic masses of atoms in the formula (1 Ca atom + 2 Cl atoms) = amu = amu Formula mass of CaCl 2

Counting Large Quantities Many chemical calculations require counting atoms and molecules Many chemical calculations require counting atoms and molecules It is difficult to do chemical calculations in terms of atoms or formula units It is difficult to do chemical calculations in terms of atoms or formula units Since atoms are so small, extremely large numbers are needed in calculations Since atoms are so small, extremely large numbers are needed in calculations Need to use a special counting unit just as used for other items Need to use a special counting unit just as used for other items A ream of paper A ream of paper One dozen donuts One dozen donuts A pair of shoes A pair of shoes

The Mole It is more convenient to use a special counting unit for such large quantities of particles It is more convenient to use a special counting unit for such large quantities of particles Mole: A unit that contains х objects Mole: A unit that contains х objects It is used due to the extremely small size of atoms, molecules, and ions It is used due to the extremely small size of atoms, molecules, and ions 6.022x10 23 particles in 1 mole 6.022x10 23 particles in 1 mole Called Avogadro’s Number Called Avogadro’s Number Periodic Table Periodic Table The average atomic mass in amu (one atom) The average atomic mass in amu (one atom) The weight of 1 mole of the element in grams The weight of 1 mole of the element in grams Avogadro’s number provides the connecting relationship between molar masses and atomic masses Avogadro’s number provides the connecting relationship between molar masses and atomic masses

Calculating the Number of Molecules in a Mole How many molecules of bromine are present in mole of bromine gas? How many molecules of bromine are present in mole of bromine gas? Given: mol Br 2 Need: molecules of Br 2 Avogadro’s number Conversion factors: Equality: Set Up Problem:

Subscripts State Moles of Elements The subscripts in a chemical formula indicate the number of atoms of each element present in a compound The subscripts in a chemical formula indicate the number of atoms of each element present in a compound The subscripts in a chemical formula can also indicate the number of moles of atoms of each element present in one mole of a compound The subscripts in a chemical formula can also indicate the number of moles of atoms of each element present in one mole of a compound i.e. In one molecule of glucose (C 6 H 12 O 6 ) there are 6 atoms of carbon, 12 atoms of hydrogen, and 6 atoms of oxygen i.e. In one molecule of glucose (C 6 H 12 O 6 ) there are 6 atoms of carbon, 12 atoms of hydrogen, and 6 atoms of oxygen

Calculating the Moles of an Element in a Compound How many moles of carbon atoms are present in 1.85 moles of glucose? How many moles of carbon atoms are present in 1.85 moles of glucose? Plan: moles of glucose moles of C atoms subscript (One) mol C 6 H 12 O 6 = 6 mols C atoms Equality: Conversion Factors: Set Up Problem: 11.1 mol C atoms

Molar Mass The atomic mass of a carbon-12 atom is amu The atomic mass of a carbon-12 atom is amu The atomic mass of one mole of carbon-12 atoms g The atomic mass of one mole of carbon-12 atoms g One mole of any element is the amount of atoms (molecules or ions) that is equal to its atomic mass (in grams) One mole of any element is the amount of atoms (molecules or ions) that is equal to its atomic mass (in grams) This mass contains х particles of that element This mass contains х particles of that element Use the periodic table to obtain the molar mass of any element Use the periodic table to obtain the molar mass of any element

Molar Mass When the number of grams (weighed out) of a substance equals the formula mass of that substance, Avogadro’s number of molecules of that substance are present When the number of grams (weighed out) of a substance equals the formula mass of that substance, Avogadro’s number of molecules of that substance are present

Molar Mass of a Compound Calculate the molar mass of iron (II) sulfate Calculate the molar mass of iron (II) sulfate Formula is FeSO 4 Formula is FeSO 4 1) Calculate the molar mass of each element 2) Each element is multiplied by its respective subscript: (number of moles of each element) 3) The molar mass is calculated by the sum of the molar masses of each element Moles of Element in Compound Moles of Compound Formula Subscript

Molar Mass of a Compound Formula is FeSO 4 : 1) Formula is FeSO 4 : The molar masses of iron, sulfur, and oxygen are 2) Multiply each molar mass by its subscript 3) Find the molar mass of the compound by adding the mass of each element

Calculations Using Molar Mass The three quantities most often calculated The three quantities most often calculated Number of particles Number of particles Number of moles Number of moles Number of grams Number of grams Using molar mass as a conversion factor is one of the most useful in chemistry Using molar mass as a conversion factor is one of the most useful in chemistry Can be used for g to mole and mole to g conversions Can be used for g to mole and mole to g conversions

Relationship between Moles, Molar Mass and Avogadro’s number Moles of substance Particles of substance Moles of substance Grams of substance Avogadro’s Number Molar Mass Moles of substance Avogadro’s Number Moles of substance Avogadro’s Number Particles of substance Moles of substance Avogadro’s Number

Converting Mass of a Compound to Moles International Foods Coffee contains 3 mg of sodium chloride per cup of coffee. How many moles of sodium chloride are in each cup of coffee? International Foods Coffee contains 3 mg of sodium chloride per cup of coffee. How many moles of sodium chloride are in each cup of coffee? Equality: 1 mol NaCl = g 3 mg NaCl moles of NaCl = g NaCl

Converting Grams to Particles Ethylene glycol (antifreeze) has the formula C 2 H 6 O 2. How many molecules are present in a 3.86 × g sample? Ethylene glycol (antifreeze) has the formula C 2 H 6 O 2. How many molecules are present in a 3.86 × g sample? Plan: convert g moles molecules of ethylene glycol Equality 1: Conversion Factor 1 Equality 2: Conversion Factor molecules Molar mass Avog Number