Stoichiometry
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Transcript Stoichiometry
Stoichiometry:
Measurement of
Elements
Formula Weights
Sum of the atomic weights (amu)
of each atom in a chemical
formula.
Called “molecular weight” or
“molecular mass” for a molecule.
Use “formula weight” for ionic
cmpds.
The Mole
Chemists have derived a special
counting unit for describing very large
numbers of atoms or molecules.
Mole (mol) = the amount of matter that
contains as many objects as the number
of atoms in exactly 12g of isotopically
pure carbon-12.
Mole, cont.
Avogadro’s number: 6.022 x 10^23
A mole of ANYTHING contains 6.022 x 10^23
objects!
I mole of marbles would cover the entire surface
of the earth in a layer ________ thick.
Pennies placed side to side in a straight line
would encircle the earth ________ times.
Representative Particles
Species present in a substance.
Substance
Type of
Species
Representative Representative
Particles
Particles in
1.00 mole
Lithium
element
atom
6.02 x 10^23
Nitrogen
gas
Calcium
fluoride
Calcium
ion
molecular
compound
ionic
compound
ion
molecule
6.02 x 10^23
formula unit
(f.u.)
ion
6.02 x 10^23
6.02 X 10^23
Moles, cont., cont.
Remember:
1 mol of carbon = 6.022 X 10^23 atoms.
1 mol water = ____ molecules.
1 mol CN- ions = _____ ions.
1 mol NaCl = ____ formula units (f.u.’s).
1 mol elephants = _____ elephants.
Mole-Particle/Particle-Mole
Problems
Use dimensional analysis!
How many moles of magnesium are
1.25 x 10^24 atoms of magnesium?
How many moles are 2.80 x 10^24 atoms Si?
How many molecules are in 3.60 mol water?
How many hydrogen atoms are in 3.60 mol
water????
Molar Mass
A mole is always the same number but
moles of different substances have
different masses.
1 mol water = 18.02 g; 1 mol lead =
207.2 g
Molar mass: the mass in grams of 1
mole of a substance. Unit is g/mol.
Calculating Molar Mass
Calculate molar mass of CH4.
C: 1 atom x 12.01 g/mol = 12.01 g/mol
H: 4 atoms X 1.01 g/mol = 4.04 g/mol
Add together:
12.01g/mol + 4.04 g/mol = 16.05 g/mol
Watch your sig figs!!!
Practice Calculating Molar Mass
Calculate the molar mass of NaCl.
Calculate the molar mass of calcium
hydroxide.
Calculate the molar mass of Sr3(PO4)2.
Do you like this more than
nomenclature?
Mass-Mole/ Mole-Mass Problems
Use dimensional analysis!
Find the mass of 4.52 x 10^-3 mol water.
Find the mass of 0.342 mol nitrogen gas.
How many moles are in 72.96 g LiCl?
How many moles are in 12 28 g KOH?
Are we having fun yet?
Mole-Volume Relationship
Avogadro’s Hypothesis: equal volumes
of gases at the same temp and pressure
contain an equal number of particles.
Note: the size or mass of gas particles
does NOT matter because particles are
so far apart from each other.
Molar Volume of a Gas
STP = standard temp and pressure
Standard temp for gases = 0°C
Standard pressure = 1 atm
“Molar volume of a gas” means that at
STP…
I mole of ANY gas = 22.4L of gas
Mole-Volume Problems
We use molar volume to do mole-volume
problems! Yeah!
Calculate the volume of 3.20 x 10^-3 mol
carbon dioxide at STP.
0.0717 L carbon dioxide
Calculate the volume of 3.70 mol nitrogen
gas at STP.
82.9 L nitrogen gas
More Molar Volume Calculations!
You have 0.200 L hydrogen gas at STP.
How many moles?
8.93 x 10^-3 mol
Dan has 0.375 mol of oxygen gas at
STP. What is its volume?
8.40 L oxygen
Density & Molar Volume at STP
The density of gases is in g/L,
NOT g/mL. WHY?
You can use density along with
molar volume to get molar
mass! Hurrah!
Density & Molar Volume Problems
A gas has a density of 3.58 g/L at STP. What
is the molar mass of the gas?
80.2 g/mol
A gas has a density of 1.964 g/L at STP. What
is its molar mass?
43.99 g/mol
What is the density of krypton gas at STP?
3.74 g/L
Percentage Composition
from Chemical Formula
Percentage by mass of each
element in a substance.
% element = (# of atoms x molar
mass of element/ molar mass of
compound) x 100
Total % of each element = 100%
Calculating Percentage
Composition
Calculate the % composition of C6H12O6.
% C = (6 x 12.01 g/mol/180.2 g/mol) x 100
= 40.00%
% H = (12 X 1.01 g/mol/180.2 g/mol) x 100
= 6.73%
% O = 100.00% - 40.00% - 6.73%
= 53.27%
Percent Composition Practice
Calculate the percent composition of
C3H8.
C: 81.8 %; H: 18.2 %
Calculate the percent on nitrogen in
NH3.
82.4% nitrogen
Percent Composition from
Mass Data
Percent Composition = (mass of
element in cmpd/mass of cmpd) x
100
Total %’s of all elements in cmpd =
100%
Percent Composition by Mass
Practice
When a 13.60-g sample of a cmpd
containing only Mg and O is
decomposed, 5.40 g oxygen is obtained.
What is the percent comp of the cmpd?
O: (5.40 g O/ 13.60 g) x 100 = 39.7%
Mg: [(13.60 g- 5.40 g)/ 13.60 g] x 100 =
60.3%
More Percent Composition
Practice
Calculate the number of grams of
sulfur in 3.54 g H2S.
First, find the % of sulfur in H2S.
94.1% S
Then, multiply .941 X 3.54 g H2S.
3.33 g
Empirical & Molecular
Formulas
Empirical formula: gives the
lowest whole number ratio of
atoms in a compound. PbO, not
Pb2O2
Molecular formula: whole number
multiples of the empirical formula.
Determining Empirical
Formulas
Sing the Empirical Formula Song
(sung to tune of “Twinkle, Twinkle
Little Star”):
“Percent to mass,
mass to mole,
divide by small,
multiply ‘til whole.”
Empirical Formula Practice
A compound is analyzed and found
to contain 25.9% nitrogen and
74.1% oxygen. What is the
empirical formula of the compound?
N2O5
More Empirical Formula
Calculations!
Find the empirical formula for the
following cmpd: Hg: 67.6%; S:
10.8%; O: 21.6%
HgSO4
Find the empirical formula for the
following cmpd: H: 5.9%; O: 94.1%
HO
Molecular Formulas
Once you have determined the empirical
formula (e.f.), you can determine the
molecular formula (m.f.) if you know the
compound’s molar mass.
Divide the molar mass of the cmpd by
the molar mass of the e.f. and get a
whole number. Multiply this number by
each subscript in the e.f. to get the m.f..
Finding a Molecular Formula
You have the empirical formula HO. Find
the molecular formula if its molar mass is
34.0 g/mol.
34.0 g/mol/ 17.0 g/mol = 2
The empirical formula HO becomes the
molecular formula H2O2 !
Finding Molecular Formulas :-)
Find the molecular formula of a cmpd
with a molar mass of 60.0 g/mol and an
empirical formula of CH4N.
C2H8N2
Find the m.f. of a cmpd with a m.m of
62.0 g/mol and an e.f. of CH3O.
C2H6O2