coefficients give ratios
coefficients of a balanced equation give the ratio of molecules in the reaction
___ H2 (g) + ___ O2 (g) ___ H2O (l)
___ molecules of hydrogen gas react with ___ molecules of oxygen gas to give ___ molecules of water
To relate coefficients to
ratiosof reactants and
products and reaction
completion.
To relate coefficients to
ratiosof reactants and
products and reaction
completion.
2 hydrogen molecules 1 oxygen molecule
2 water molecules
Correct Ratio of molecules 2H2 : 1O2 : ?H2O
1 hydrogen molecule 1 oxygen molecule
?
Wrong Ratio of molecules 1H2 : 1O2 : ?H2O
BUT…Can you “measure out” 2 molecules of hydrogen and 1 molecule of water in the lab???
Why or why not???
What unit do we use to measure in the lab?
How many molecules are in one gram?
Since one gram contains a huge amount of molecules or atoms...
chemists have devised a special counting unit to understand the large numbers of molecules or atoms measured in the lab.
This unit is called
the mole!
What is a “counting unit”?
• Bakers use the counting unit “dozen” to understand they have 12 donuts
• Pencils are sold by the “gross”. If you buy 1 “gross” you will get 144 pencils.
• give relationships between units
LiNkEd
If you had 9454 pencils; calculate how many gross you would have. (remember 1 gross = 144 pencils)
Practice using a graphic organizer
Practice – using relationships to give
Conversion Fractions
How many eggs are in 3.5 dozen?
If you have 7.3 gross of pencils; how many individual pencils would you have ?
What is a mole?
• a “mole” is the counting unit for particles in chemistry
• A mole is the amount of matter (grams) that as many particles as there are numbers of atoms in exactly 12g of isotopically pure 12C
But how much is that???
To relate moles of a substance
to the number of particles
To relate moles of a substance
to the number of particles
LiNkeD
Mole Facts
• Scientists have experimentally determined that 12g of 12C is made up of 6.02 x 1023 atoms of 12C
• Scientists call 6.02 x 1023 Avogadro’s Number
LINKED
So…Examples1 mol of H2O = 6.02 x 1023 molecules
H2O1 mol of NO3
- =
1 mol of Al =
1 mol of rice =
(this much rice would cover all the land area of the Earth to a depth of about 75 meters
of rice)
6.02 x 1023 ions of NO3-
6.02 x 1023 atoms Al
6.02 x 1023 grains of rice
It would take over 19 million years to spend
Avogadro's number of dollars
if the money were spent
at the rate of one billion dollars per
second.
Calculate how many sodium atoms are in 0.120 mole of Na.
How many moles of potassium is 1.25 x 1021 atoms of K?
Practice
Molar Mass (MM)
Since 1mol always has the 6.02x1023 particles, and (by definition) 1mol 12C has a mass of 12g…
1mol of 24Mg must have a mass of
1 atom of 12C = 12amu 1mol 12C = 12grams
1atom of 24Mg = 24amu 1mol 24Mg = 24grams
24g
To state the mass of
Avogadro’s number of
atoms for any element
referring to the periodic
table.
To state the mass of
Avogadro’s number of
atoms for any element
referring to the periodic
table.
Molar Mass (MM)The mass of a single atom of an element (in amu) is numerically equal to the mass (in grams) of 1mol of that element
The molar mass of an element is equal to its atomic mass (in grams)
1 mol Zn = _____ g
1mol Al = _____ g
How many atoms of Zn are in 2.3 mole of Zn?
Determine the # of moles 4.39 x 1034 atoms Al makes up.
Practice
What is the mass (in grams) of 2.01 x 1022 atoms of S?
How many atoms make up 2.34g Ca?
Practice
Molar Massof a compound is calculated by adding the
molar masses of each element
What is the Molar Mass Fe2O3 ???
Molar Mass of… Fe :1 mol Fe = 55.85g Fe O : 1 mol O = 16g O
So, for Fe2O3 :
2(55.85g Fe) + 3 (16.00 g O) = 159.7g/mol
To calculate the molar mass of a substance given its chemical formula.
To calculate the molar mass of a substance given its chemical formula.
How many grams does 1 mole of Mg(NO3)2
weigh?
How many grams does 2.39 mole Mg(NO3)2
weigh?
Practice
Practice
How many N atoms are there in 4.31g of Mg(NO3)2 ?
How many moles is 13.45 g of Mg(NO3)2 ?
What happens when chemists make new compounds in the lab???
Synthetic chemist – makes new compounds
Analytical chemist – “analyzes” new compound to give experimental evidence of its Composition and formula
Percent Composition
Empirical Formula
Molecular Formula
Percent Composition
Empirical Formula
Molecular Formula
Percent Composition
Analytical chemist determines that a 100 g sample of a new compound contains 55g of element X and 45 g of element Y. What is the percent composition by mass of each element?
To calculate the percent
composition of a compound
given its chemical formula.
To calculate the percent
composition of a compound
given its chemical formula.
Percent CompositionDetermine the percent by mass of each element in sodium hydrogen carbonate.((The sum of the percents should be 100%))
Known Unknownformula = NaHCO3 % Na = ?
% H = ?% C = ?% O = ?
Assume you have a total of one mole
NaHCO3
Empirical Formula
• is the simplest whole-number ratio of elements present
• may or may not be the same as the molecular formula
Example : “hydrogen peroxide” empirical formula = HOmolecular formula = H2O2
To calculate empirical
formula of a compound given its percent
composition
To calculate empirical
formula of a compound given its percent
composition
Empirical Formula
The % composition of an oxide of sulfur is 40.05% S and 59.95% O. Determine the empirical formula.1. Assume total mass = 100g2.Thus given percents = masses • 40.05g S and 59.95g O
3.Convert mass to moles of each – (this gives the mole ratio AND MUST be whole
numbers in order to be used as subscripts!!)
4.Element with the smaller # of moles will have smallest possible subscript of 1
Empirical FormulaMethyl Acetate is a solvent commonly used in some paints, inks, and adhesives. Determine the empirical formula of methyl acetate, which has the following chemical analysis:
48.64% carbon8.16% hydrogen43.20% oxygen
Molecular Formula• specifies actual # of atoms of each
element in one molecule (or formula unit)• To calculate:
“n” being the integer used to multiply by to get molecular formula
To calculate molecular
formula of a compound given its empirical formula.
To calculate molecular
formula of a compound given its empirical formula.
Molecular Formula
The empirical formula for fructose (fruit
sugar) is CH2O. The experimentally determined molar mass is 180g/mole, find the actual molecular formula for the sugar.
Stiochiometry
• relationship of quantities (mass or volume) in a chemical reaction according to the coefficients of a balanced equation
• used to “determine” quantities of reaction (“how much” of one substance will react to make “how much” another substance)
To relate the coefficients in
a balanced equation to
moles of reactant &
product
To relate the coefficients in
a balanced equation to
moles of reactant &
product
Linked
Example: SMOG NO2
NO + O2 NO2
“2 molecules NO react with 1 molecule of O2 to give 2 molecules NO2”
OR…“2000 molecules NO react with 1000 molecules O2 to give 2000 molecules NO2”
OR… “ 2(6.02x1023)molecules NO react with
6.02x1023 molecules O2 to give 2(6.02x1023) molecules NO2”
2 2
Linked
LINKED
Stiochiometry
Remember that
6.02x1023 molecules = 1 mole of that substance so…
2 mole NO + 1 mole O2 2 mole NO2
Coefficients of a balanced equation indicate
mole ratio of reactants and products
To relate the number of
moles of two substances in a balanced equation.
To relate the number of
moles of two substances in a balanced equation.
PracticeIn the 19th Century, mercury (II) iodide was used as a treatment for syphilis; a reaction that produces Mercury (II) Iodide is:
KI (aq) + Hg(NO3)2 (aq) HgI2 (s) + KNO3 (aq)
Dosages were very small, to make a 0.52g dose of mercury (II) iodide using this reaction how much potassium iodide would
you need to start with?
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