Mass:≈ 1.66 × 10 −27 to 4.52 × 10 −25 kg The mole is a measure of the amount of a...

Mass: ≈ 1.66 × 10−27 to 4.52 × 10−25 kg

The mole is a measure of the amount of a substance.

Abbreviated mol Amount of substance which contains as

many particles as there are atoms in exactly 12 g of 12C.

Avogadro’s number = 6.02 x 1023

A mole of dollars was created at the time the earth was formed, approximately 4.5 billion years ago. It was givenaway at the rate of exactly 1 million dollars per second since that time. Did the money run out?

Mole conversions:

To calculate the number of atoms (molecules, formula units in a given number of moles, multiply by Avogadro’s number.

To calculate the number of moles in a given number ofatoms (molecules, formula units) divide by Avogadro’s number.

Mathematically defined as the mass of the substance divided by its molar mass.

Molar mass = the sum of the weights of each of the atoms in the compound. Ex. NaCl = 22.99 + 35.45 = 58.44 g

Determine the molar mass of: CO2, NH3, sodium sulfate, barium chloride.

How Big Is A Mole?

If someone had a mole of pennies and wanted to buy kite string at a million dollars/in2,they would have enough string toloop around the earth 3 million times and go from the earth tothe moon 25000X.

Calculate the number of moles in the following:

5.46 g calcium nitrate

3.99 g aluminum hydroxide

15.5 g magnesium phosphide

Percentage Composition Empirical Formulas Molecular Formulas

The percentage by mass contributed by each element in the substance.

Often used to verify the purity of a compound.

Uses formula weights from the periodic table if the formula is known.

% elem. = total weight of element formula weight of compound x

100%

Calculate the percentage composition of sucrose (C12H22O11):

%C = (12) (12.01) %O = (11) (16)342.34 342.34

%H = (22) (1.01)342.34

For each of the following, find the percentagecomposition from the analysis determined byexperiment.

1.A 14.80 g sample contains 3.83 g Fe and 10.97 g Br.

2. A 9.14 g sample contains 4.77 g C, 1.19 gH, and 3.18 g O.

Calculate the percentage composition from theformula of the compound.

Sodium iodide

Calcium oxalate

Aluminum Sulfate

Indicates the relative number of atoms of each element in a compound.

It is the smallest whole number ratio of the atoms in a formula.

Gives the subscripts for the formula.

Empirical Formulas are determined from experimental results.

A compound contains 40.92% C, 4.58% H, and 54.50% O. What is the empirical formula of the compound?

1.Assume a 100 gram sample so that the percentages become grams.

2. Convert the grams to moles.

3. Divide by the smallest number.

4. You may not round values that are > .2 or < .9. Multiply by some factor to makethe decimal a whole number.

The formulas derived from percentagecomposition are always empirical formulas.

The molecular formula represents the actual numberof atoms in a formula.

It is always a multiple of the empirical formula.

CH4 is an empirical formula; C2H8 is a molecular formula. The two are related by their molar masses.

CH4 = 16.05 g/mol C2H8 = 32.10 g/mol 32.10/16.05 = 2

The two tells you to multiply the empirical formula’ssubscripts by 2.

Mesitylene has an empirical formula of C3H4. Themolecular mass for this compound is 121 g. Determine the molecular formula for mesitylene.

Ethylene glycol, used in antifreeze, is composed of38.7% C, 9.7% H and 51.6%O. The molar mass ofethylene glycol is 62.1 g/mol. What is the molecularformula for the compound?

Mini-Review: Empirical Formulas

Some crystals have water molecules as part oftheir lattice; these are called hydrates.

CuSO4.5H2O

Analyzing these compounds requires driving off the water that is present and weighing the “anhydrous” or dry salt produced.

A 5.00 g sample of a hydrate of barium chloride isheated. The remaining salt is found to weigh 4.26 g. Determine the empirical formula of this hydrate.