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

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 !

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