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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
SpeciesRepresentative
Particles Representative
Particles in 1.00 mole
Lithium element atom 6.02 x 10^23
Nitrogen gas
molecular compound
molecule 6.02 x 10^23
Calcium fluoride
ionic compound
formula unit (f.u.)
6.02 x 10^23
Calcium ion
ion ion 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/molH: 4 atoms X 1.01 g/mol = 4.04 g/molAdd 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 pressureStandard temp for gases = 0°CStandard 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 dioxideCalculate 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 FormulaPercentage 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 CompositionCalculate 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 PracticeCalculate 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 FormulasEmpirical 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 FormulasSing 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 !