StoichiometryStoichiometry

(part II)(part II)

1 mole of anything

= 6.022 x 1023 units of that thing (Avogadro’s number)

= molar mass of that thing

Thus, the molar mass of an element is numerically equal to its atomic mass (reflected on the periodic table)

molar mass = atomic mass

Review Slide:

Mass percent of an element:

• Calculate the mass % of Fe in iron(III) oxide, (Fe2O3):

Percent Composition of Compounds

Review Slide:

Review Problem:

In 1987 the first substance to act as a superconductor at a temperature above that of liquid nitrogen (77K) was discovered. The approximate formula of this substance is YBa2Cu3O7.

Calculate the percent composition by mass of this material

Empirical formula - Simplest whole-number ratio

> Base the calculation on 100g of compound.

> Determine the number of moles of each element present in 100g of compound using the atomic masses of the elements present.

> Divide each value of the number of moles by the smallest of the values.

* If each resulting number is a whole number (after appropriate rounding), these numbers represent the subscripts of the elements in the empirical formula.

* If the numbers obtained in the previous step are not whole numbers, multiply each number by an integer so that the results are all whole numbers

Formulas

Molecular formula Actual formula of the compound

= (empirical formula)n

where:

n => integer = molar mass

empirical formula mass

Formulas

•7

e.g. #1

The composition of adipic acid is 49.3% C, 6.9% H, and 43.8% O (by mass). The molar mass of the compound is about 146 g/mol.

What is the empirical formula? What is the molecular formula?

Chemical Stoichiometry

Stoichiometry – The study of quantities of materials consumed and produced in chemical reactions.

Stoichiometric Calculations

• Chemical equations can be used to relate the masses of reacting chemicals.

Review Slide:

• The number of atoms of each type of element must be the same on both sides of a balanced equation.

• Subscripts must not be changed to balance an equation.

• A balanced equation tells us the ratio of the number of molecules which react and are produced in a chemical reaction.

• Coefficients can be fractions, although they are usually given as lowest integer multiples.

Notes on Balancing Chemical EquationsReview Slide:

Limiting reactant – the reactant that is consumed first and therefore limits the amounts of products that can be formed.

• Determine which reactant is limiting to calculate correctly the amounts of products that will be formed.

Limiting ReactantsReview Slide:

• An important indicator of the efficiency of a particular laboratory or industrial reaction.

Reaction Yield

% yield

= (actual yield / theoretical yield) x 100%

Review Slide:

• For 1 mole of an ideal gas at 0°C and 1 atm, the volume of the gas is 22.42 L.

• STP = standard temperature and pressure 0°C and 1 atm Therefore, the molar volume is 22.42 L

at STP.

Molar Volume of an Ideal Gas

V = nRT =

P

(1 mol) 0.08206 L atm (273.2K) = 22.42L mol K

1 atm

Calculate the volume that 0.881 moles of oxygen gas at STP will occupy.

e.g. #2

D = density of gas T = temperature in Kelvin P = pressure of gas R = universal gas constant

Molar Mass of a Gas

MM = Molar Mass = DRT

P

What is the density of F2 at STP (in g/L)?

e.g. #3

- concentration of solutions

- expressed as molar concentrations in M (mol/L)

Molarity (M)

Oxalic Acid (H2C2O4) is used for the removal of rust (Fe2O3). Calculate the number of grams of rust that can be removed with 1 L 0f 0.10M solution of oxalic acid. The equation is shown as:Fe2O3 + H2C2O4 => Fe(C2O4)3 + H2O + H

e.g. #4a

Which is the limiting reactant if Fe(C2O4)3 is our concerned product in e.g. #4a?

e.g. #4b

e.g. #4c

What mass of product will be produced in e.g. #4a?

e.g. #4d

What would the % yield of the reaction on e.g.#4a if only 2.5g of Fe(C2O4)3 are actually isolated at the end?