It’s a beauty mark… It’s a small furry garden pest… No, wait… its how we count ATOMS!

It’s a beauty mark…

It’s a small furry garden pest…

No, wait… its how we count ATOMS!

The MOLE

The mass of 1 mole of a substance.

Molar Mass

Calculate the molar mass of 1 mole of magnesium chloride. (first you need a formula)

Calculate the molar mass

FORMULAS - review

MgCl2The subscript is the number at the bottom of a formula.There is 1- Mg & 2 - Cl

How to calculate molar mass

1. Identify the # of atoms of each element

2. Multiply # atoms by the atomic mass of that element. (round to 2 #’s after the decimal)

3. Add them all together

4. Grams (g) is the unit

Molar MassMgCl2

Mg – 1 (24.31) = 24.31Cl – 2 (35.45) = 70.90

or

95.21 g/mol

95.21 g MgCl2

1 mole MgCl2

=

•Molar mass allows scientist a way to measure moles in a lab.

•Molar mass of Fe =

•Molar mass of O2 =

•Molar mass of Cu(OH)2=

Molar Mass

Calculate the molar mass: (you MUST write the formula

correctly before answering)

1 mole of Ammonium phosphide

1 mole of Trinitrogen pentachloride

•The percent BY MASS of each element in a compound – divide the element’s total mass (part) by the molar mass (whole) then multiple by 100 to get the percent.

•Ex: % composition of MgCl2

Mg – 1 (24.31) = 24.31 / 95.21 x 100 = 25.53% Mg Cl – 2 (35.45) = 70.90 / 95.21 x 100 = 74.47% ClMolar mass = 95.21 g/mol

Percent Composition

(PART)

(WHOLE)

(PART)

(WHOLE)

K – 1 (39.10) = 39.10 / 158.04 x 100 =

24.74% K Mn – 1 (54.94) = 54.94 / 158.04 x

100 = 34.76% Mn

O – 4 (16.00) = 64.00 / 158.04 x 100 =

40.50% O

molar mass KMnO4 = 158.04

Practice - Calculate the % comp of KMnO4:

1. Find the % comp of the element in the compound

2. Change the % to a decimal (move decimal 2 times to the left or divide by 100)

3. Multiply that decimal by the amount (g) of the sample.

Ex: Calculate amount of chlorine in 203.5 grams of MgCl2. (use the % we found earlier)

74.47% Cl = .7447 x 203.5 = 151.5 grams Cl

Calculating the amount of an element in a sample

H – 2(1.01) = 2.02

O – 1(16.00) = 16.00

Molar mass H2O = 18.02

Practice: Calculate the amount of oxygen in 15.75 grams of water.

/ 18.02 x100 = 88.79% O

88.79% O = .8879 x 15.75 = 13.98 grams O

Complete % comp worksheet

•The lowest whole number ratio (subscripts) of elements in a compound.

•Cannot be reduced!!!

not empirical empirical

•Ex: C6H12O6 CH2O

Empirical Formula

• Actual number of atoms in a chemical compound

molecular

• EX: C12H24O12

• Molecular Formulas can be reduced to Empirical Formulas

molecular empirical

• EX: C12H24O12 CH2O

• Different molecular formulas can have similar empirical formulas

• molecular empirical

• EX: N3O9

N12O36

Molecular Formula

Molecular formula: C76H52O46

Empirical formula: ___________NO3

1. C2H4

2. NO3

3. S9Cl12

4. C3Cl9

5. N4S9

PRACTICE:1. Identify each as empirical (can’t be

reduced) or molecular (can be reduced)

2. If its molecular – write the empirical

molecular

molecular

molecular

empirical

empirical

CH2 - empirical

S3Cl4 - empirical

CCl3 - empirical

• Ex: A compound was found to be 54.53% Carbon, 9.15% Hydrogen, and 36.32% Oxygen. Find its Empirical Formula.

Steps:•Assume a 100g sample (change % g)

•Use molar mass to find moles of each

•Divide all moles by the smallest number of moles

•Round each to the nearest whole # (sometimes you have to multiply to get a whole number - special)

• The resulting whole #are the subscripts for that element in the empirical formula

Finding Empirical Formula from Percent Composition

63.5% Silver 8.2% Nitrogen 28.3% Oxygen

63.5 g Ag 8.2 g N 28.3 g O

107.87 14.01 16.00

.589 mole Ag .59 mole N 1.77 mole O

.589 .589 .589

1 1 3

AgNO3

Calculating Empirical Formula

60.00%C 4.48%H 35.53%O

60.00g C 4.48g H 35.53g O

12.01 1.01 16.00

4.996 mole C 4.44 mole H 2.221 mole N

2.221 2.221 2.221

2.249 2 1x4 x4 x4

9 8 4

C9H8O4

Calculating Empirical Formula (special)

1.Find the empirical formula

2.Calculate the molar mass of your empirical formula

3.Identify the molar mass of your molecular (GIVEN in the problem everytime!)

4.Divide the molecular mass / empirical mass

5.Round to the nearest whole #

6.Multiply the whole # by the subscripts in the Empirical formula

Calculating Molecular Formula

If a compound has an empirical formula of NO3

and a molecular mass of 186g – what is the molecular formula?

Empirical formula: NO3 molar mass: 62.01g

Molecular mass (given) 186g

empirical mass 62.01

3 x NO3 = N3O9

Practice

•For counting matter (quantity) in chemistry we use the mole•Moles are used to correctly measure chemicals for reactions in a lab.•Problem – no way to “physically” measure a mole. (no lab equipment measures moles)•Solution – molar mass (balance)

What is a mole?

Remember – Molar mass is the mass (grams) of 1 mole

1 mole Fe = _________grams Fe

2.5 mole Fe = ________ grams Fe

113.5 grams Fe = _______ moles Fe

Mass to mole = divide by molar mass

Mole to mass = moletiply by mole mass

Using Molar Mass

How many grams are in 15.7 mole MgCl2

How many moles are in 0.75 grams of silver?

What is the mass of 30.7 mole water?

Using molar mass

In the lab, Mrs. Mathieson needs 2.57 moles of NaCl to do an experiment. How many grams would be needed to equal 2.57 moles of NaCl?

After doing the experiment, Mrs. Mathieson has 1.02 moles of NaCl remaining – how many grams does that equal?

Using molar mass

•For counting matter (quantity) in chemistry we use the mole

•1 mole = 6.02 x 1023 representative particles (particles are very tiny)

•This value is called Avogadro’s Number

What is a mole?

• ie. the smallest particle that retains chemical and physical properties

•3 types depending on the compound:•Atoms: Single element •Molecules: covalent compound •Formula Units: Ionic Compounds or ions

What are REPRESENTATIVE PARTICLES?

Remember there are 6.02 x 1023 particles (atoms, molecule, f.u) in 1 mole

How many atoms are in 1 mole?

How many atoms are in 2.10 moles of Copper?

How many moles are in 4.21 x 1026 atoms of aluminum?

Using Avagadro’s Number

•For counting matter (quantity) in chemistry we use the mole

•1 mole = 22.4 L of any gas

•This value is called Molar Volume

What is a mole?

How many liters are in 1 mole of any gas?

How many liters are in 12.95 L of oxygen gas?

How many moles are in 0.758 moles of nitrogen gas?

Using molar volume