coefficients give ratios

coefficients give ratios

coefficients of a balanced equation give the ratio of molecules in the reaction

___ H2 (g) + ___ O2 (g) ___ H2O (l)

___ molecules of hydrogen gas react with ___ molecules of oxygen gas to give ___ molecules of water

To relate coefficients to

ratiosof reactants and

products and reaction

completion.

To relate coefficients to

ratiosof reactants and

products and reaction

completion.

2 hydrogen molecules 1 oxygen molecule

2 water molecules

Correct Ratio of molecules 2H2 : 1O2 : ?H2O

1 hydrogen molecule 1 oxygen molecule

?

Wrong Ratio of molecules 1H2 : 1O2 : ?H2O

BUT…Can you “measure out” 2 molecules of hydrogen and 1 molecule of water in the lab???

Why or why not???

What unit do we use to measure in the lab?

How many molecules are in one gram?

Since one gram contains a huge amount of molecules or atoms...

chemists have devised a special counting unit to understand the large numbers of molecules or atoms measured in the lab.

This unit is called

the mole!

What is a “counting unit”?

• Bakers use the counting unit “dozen” to understand they have 12 donuts

• Pencils are sold by the “gross”. If you buy 1 “gross” you will get 144 pencils.

• give relationships between units

LiNkEd

If you had 9454 pencils; calculate how many gross you would have. (remember 1 gross = 144 pencils)

Practice using a graphic organizer

Practice – using relationships to give

Conversion Fractions

How many eggs are in 3.5 dozen?

If you have 7.3 gross of pencils; how many individual pencils would you have ?

What is a mole?

• a “mole” is the counting unit for particles in chemistry

• A mole is the amount of matter (grams) that as many particles as there are numbers of atoms in exactly 12g of isotopically pure 12C

But how much is that???

To relate moles of a substance

to the number of particles

To relate moles of a substance

to the number of particles

LiNkeD

Mole Facts

• Scientists have experimentally determined that 12g of 12C is made up of 6.02 x 1023 atoms of 12C

• Scientists call 6.02 x 1023 Avogadro’s Number

LINKED

So…Examples1 mol of H2O = 6.02 x 1023 molecules

H2O1 mol of NO3

- =

1 mol of Al =

1 mol of rice =

(this much rice would cover all the land area of the Earth to a depth of about 75 meters

of rice)

6.02 x 1023 ions of NO3-

6.02 x 1023 atoms Al

6.02 x 1023 grains of rice

It would take over 19 million years to spend

Avogadro's number of dollars

if the money were spent

at the rate of one billion dollars per

second.

Calculate how many sodium atoms are in 0.120 mole of Na.

How many moles of potassium is 1.25 x 1021 atoms of K?

Practice

Molar Mass (MM)

Since 1mol always has the 6.02x1023 particles, and (by definition) 1mol 12C has a mass of 12g…

1mol of 24Mg must have a mass of

1 atom of 12C = 12amu 1mol 12C = 12grams

1atom of 24Mg = 24amu 1mol 24Mg = 24grams

24g

To state the mass of

Avogadro’s number of

atoms for any element

referring to the periodic

table.

To state the mass of

Avogadro’s number of

atoms for any element

referring to the periodic

table.

Molar Mass (MM)The mass of a single atom of an element (in amu) is numerically equal to the mass (in grams) of 1mol of that element

The molar mass of an element is equal to its atomic mass (in grams)

1 mol Zn = _____ g

1mol Al = _____ g

How many atoms of Zn are in 2.3 mole of Zn?

Determine the # of moles 4.39 x 1034 atoms Al makes up.

Practice

What is the mass (in grams) of 2.01 x 1022 atoms of S?

How many atoms make up 2.34g Ca?

Practice

Molar Massof a compound is calculated by adding the

molar masses of each element

What is the Molar Mass Fe2O3 ???

Molar Mass of… Fe :1 mol Fe = 55.85g Fe O : 1 mol O = 16g O

So, for Fe2O3 :

2(55.85g Fe) + 3 (16.00 g O) = 159.7g/mol

To calculate the molar mass of a substance given its chemical formula.

To calculate the molar mass of a substance given its chemical formula.

How many grams does 1 mole of Mg(NO3)2

weigh?

How many grams does 2.39 mole Mg(NO3)2

weigh?

Practice

Practice

How many N atoms are there in 4.31g of Mg(NO3)2 ?

How many moles is 13.45 g of Mg(NO3)2 ?

What happens when chemists make new compounds in the lab???

Synthetic chemist – makes new compounds

Analytical chemist – “analyzes” new compound to give experimental evidence of its Composition and formula

Percent Composition

Empirical Formula

Molecular Formula

Percent Composition

Empirical Formula

Molecular Formula

Percent Composition

Analytical chemist determines that a 100 g sample of a new compound contains 55g of element X and 45 g of element Y. What is the percent composition by mass of each element?

To calculate the percent

composition of a compound

given its chemical formula.

To calculate the percent

composition of a compound

given its chemical formula.

Percent CompositionDetermine the percent by mass of each element in sodium hydrogen carbonate.((The sum of the percents should be 100%))

Known Unknownformula = NaHCO3 % Na = ?

% H = ?% C = ?% O = ?

Assume you have a total of one mole

NaHCO3

Empirical Formula

• is the simplest whole-number ratio of elements present

• may or may not be the same as the molecular formula

Example : “hydrogen peroxide” empirical formula = HOmolecular formula = H2O2

To calculate empirical

formula of a compound given its percent

composition

To calculate empirical

formula of a compound given its percent

composition

Empirical Formula

The % composition of an oxide of sulfur is 40.05% S and 59.95% O. Determine the empirical formula.1. Assume total mass = 100g2.Thus given percents = masses • 40.05g S and 59.95g O

3.Convert mass to moles of each – (this gives the mole ratio AND MUST be whole

numbers in order to be used as subscripts!!)

4.Element with the smaller # of moles will have smallest possible subscript of 1

Empirical FormulaMethyl Acetate is a solvent commonly used in some paints, inks, and adhesives. Determine the empirical formula of methyl acetate, which has the following chemical analysis:

48.64% carbon8.16% hydrogen43.20% oxygen

Molecular Formula• specifies actual # of atoms of each

element in one molecule (or formula unit)• To calculate:

“n” being the integer used to multiply by to get molecular formula

To calculate molecular

formula of a compound given its empirical formula.

To calculate molecular

formula of a compound given its empirical formula.

Molecular Formula

The empirical formula for fructose (fruit

sugar) is CH2O. The experimentally determined molar mass is 180g/mole, find the actual molecular formula for the sugar.

Stiochiometry

• relationship of quantities (mass or volume) in a chemical reaction according to the coefficients of a balanced equation

• used to “determine” quantities of reaction (“how much” of one substance will react to make “how much” another substance)

To relate the coefficients in

a balanced equation to

moles of reactant &

product

To relate the coefficients in

a balanced equation to

moles of reactant &

product

Linked

Example: SMOG NO2

NO + O2 NO2

“2 molecules NO react with 1 molecule of O2 to give 2 molecules NO2”

OR…“2000 molecules NO react with 1000 molecules O2 to give 2000 molecules NO2”

OR… “ 2(6.02x1023)molecules NO react with

6.02x1023 molecules O2 to give 2(6.02x1023) molecules NO2”

2 2

Linked

LINKED

Stiochiometry

Remember that

6.02x1023 molecules = 1 mole of that substance so…

2 mole NO + 1 mole O2 2 mole NO2

Coefficients of a balanced equation indicate

mole ratio of reactants and products

To relate the number of

moles of two substances in a balanced equation.

To relate the number of

moles of two substances in a balanced equation.

PracticeIn the 19th Century, mercury (II) iodide was used as a treatment for syphilis; a reaction that produces Mercury (II) Iodide is:

KI (aq) + Hg(NO3)2 (aq) HgI2 (s) + KNO3 (aq)

Dosages were very small, to make a 0.52g dose of mercury (II) iodide using this reaction how much potassium iodide would

you need to start with?