Stoichiometry and Stoichiometry and the MOLE!!the MOLE!!

By: Claire Chrisman By: Claire Chrisman and Allison Wellsand Allison Wells

The Mole!

A mole is equal to 6.02 x 1023

Discovered by Avogadro Chosen because 6.02 x 1023 atoms of an element have a

mass that is equal to the element's atomic mass.

the # of moles = (the # of atoms / 6.02 x 1023 ) Mols=mass/molar mass Units= g/mol

Percent Composition

Percent Composition is the mass percent of each element in a compound.

% Element A = (Mass of element A / total mass of sample) x 100

Another formula that is useful if you are given the formula of the compound is:

% Element A= (mass of A / molar mass of compound) x 100

Stoichiometry!

1. Balance the equation.

2. Convert to moles.

3. Then multiply by (wanted / given)

4. Ta-Da! It's really that easy.

Stoichiometry example problem

_Na2CO

3 (s) _

Na

2O

(s) + _CO

2(g)

a) Balance the equation.

b) If you start with 47.2g of sodium carbonate, how many moles of sodium oxide can you make? What mass is this?

Atomic Mass

Atomic mass is the average mass of all an element's atoms.

An isotope occurs when the same element has a different number of neutrons.

To find atomic mass: (% abundance of 1st isotope)(mass of 1st isotope)(% abundance of 2nd isotope)(mass of 2nd isotope)......

Now for an example:

If the abundance of 6Li (6.015121 amu) is 7.500% and the abundance of 7Li (7.016003 amu) is 92.500%, what is the average atomic mass?

(a) 6.0750 amu

(b) 6.0902 amu

(c) 6.9250 amu

(d) 6.9409 amu

Atomic Number

Abbreviated, Z. The number of

protons in an element.

The number of protons and neutrons in an element.

Mass Number

Limiting Reactants

The limiting reactant is the reactant that is used up first in the reaction causing the reaction to stop (sometimes.)

Cool....now how do I find a limiting reactant?

Simple, you simply assume that each reactant is limiting and do some calculations to see how much product each one will make.For example:

16 Ag(s) + S8(s)→ 8 Ag

2S(s)

What mass of Ag2S can be produced from 2.0g Ag and 2.0g S

8?

Now we do stoich to find the mass of Ag2S that is made from 2.0g Ag.

Then we do stoich again to find the mass of Ag2s that is made from 2.0g

of S8.

Whichever mass is smaller will be the limiting reactant.

Carbon disulfide and oxygen react to yield carbon dioxide and sulfur dioxide. If 1 mole of CS2 is combined with 1 mole of oxygen, what is the limiting reactant? A. carbon disulfide ?

B. Oxygen ?

C. sulfur dioxide ?

D. carbon dioxide

Multiple Choice

Percent Yield

The measure of how efficient you and the reaction were at producing your product.

% Yield=(Actual Yield / Theoretical Yield) x 100 Actual=mass measured in lab

Theoretical=mass calculated from stoich

Empirical and Molecular Formulas

Empirical Formula- the most reduced form of the formula.

ex. CH2O

Molecular Formula- the formula that shows what the compound actually is.

ex. C6H

2O

6

Finding empirical formulas from molecular.

Just reduce the subscripts.

1. Convert to moles.

2. Divide all amounts by the smallest amount.

3. The numbers you get will be the subscripts.

Finding Empirical from empirical data.

Ex. If a given sample contains 87.5% N and 12.5% H by mass, find the empirical formula.

Finding molecular formulas from empirical formulas.

Pretty simple, just divide the molar mass given to you by the molar mass of the empirical formula.

ex. Styrene has an empirical formula of CH. If styrene has a molar mass of 104g/mol, what is its molecular formula?

Hydrates

A hydrate is a compound that has whole water molecules bonded to its structure.

Can be heated to remove the water, which leaves the anhydrous compound behind.

Can solve them in a way similar to empirical formulas

ex. 2.681g hydrated NiSO4 *xH

2O is heated. Anhydrous NiSO

4 has

a mass of 1.549g. Determine the hydration.

1. Subtract 2.681-1.549 to find the mass of H2O. Then convert

both 1.549 and the 1.132 to moles.

2. Next divide the moles of NiSO4 by moles H

2O.

The End!