SCH 3U Moles Name: _________________________

Avogadro’s Number, Moles and Molar Mass

For a scientist to carry out experiments involving only 1, 2, 3, a dozen or even a million atoms or molecules is not

feasible. The volume that this many atoms or molecules would occupy is negligible. Reasonable masses of elements for

scientists to work with are those that are based on the average atomic mass for an element. It was determined by the

Italian scientist, Avogadro, that the number of atoms in exactly 12.000 g of the carbon-12 isotope was 6.02 x 1023. In fact

a sample of any element equal to its average atomic mass, contains 6.02 x 1023 atoms of that element. Scientists refer to

this amount of any entity as a “mole” and use this term in the same manner that a baker would use the unit of a dozen.

The mole (symbol “n”, unit mol) is described as the number of particles in exactly 12.000 grams of . The

number 6.02 x 1023 is called Avogadro’s number and is represented by the symbol “NA”. Avogadro’s number is so big it is

in comprehensible, below are some examples to help grasp the magnitude of this number.

1. If 1 million ants live in an ant hill 1 m2, 1 mole of ants would occupy enough ant hills to cover Earth 1000 times.

2. To travel a distance of 1 mole of millimetres at the speed of light (3.0 x108 m/s) it would take 63 654 years.

The mass of 1 mole of any substance is referred to as it Molar Mass (symbol “MM”, unit g/mol) and for an

element is equal to the atomic mass on the periodic table in g/mol (for a specific isotope, the mass number is used). The

molar mass of a compound is equal to the algebraic sum of the atomic masses for each element in the formula (multiply

each element’s mass by the number of that element in the formula, then sum).

Calculations Using the Mole

There are 2 formulas for calculating the number of moles or required quantities (number of particles or mass).

Formula 1: Converting number of particles to moles

where, n= # of moles

NA= Avogadro’s #

Formula 2: Converting mass into moles

where, m = mass in grams

MM = molar mass in g/mol

Examples:

1. Determine the moles of Ca in 7.56x1024 atoms?

2. How many CO2 molecules are in 3.25 mol?

3. How many moles are in 12.75 g of sodium?

4. Calculate the mass of 2.50 mol of Mg3(PO4)2.

(

) (

)

MM = 262.87 g/mol

(

)

SCH 3U Moles Name: _________________________ Exercise:

Complete the following on a separate piece of paper and express the answer to the correct number of significant digits.

1. State the mass of 1 mole of the following in g/mol.

a)

b) nitrogen

c)

d) S

e)

f) thallium

2. Calculate the molar mass of the following compounds.

a) HBr

b) NO2

c) Cu

d) LiOH

e) Mg(OH)2

f) H3PO4

g) Fe2(SO4)3

h) C2H6O

i) C12H26

j) Sn3(PO3)4

3. Determine the number of moles in the following.

a) 6.02 x 1023 atoms of N

b) 4.52 x 1023 electrons

c) 3.011 x 1024 molecules of H2O

d) 4.25 x 1025 Ca2+ ions

4. Calculate the number of particles in the following.

a) 1.50 mol of Ba atoms

b) 3 mol of C atoms

c) 0.75 mol of H2O molecules

d) 2.67 mol of NaOH molecules

5. Determine the number of atoms in the following.

a) 1.30 mol of calcium hydroxide

b) 0.75 mol of Au2(SO4)3

c) 2.25 mol of Cr(OH)3

d) 0.46 mol of SnF4

6. Find the mass of the following.

a) 3.6 mol of Zn atoms

b) 0.25 mol of Se atoms

c) 0.43 mol of He atoms

d) 4.25 mol of Mg atoms

e) 2 mol of CBr4

f) 3 mol of Cl2

g) 3.6 mol of Zn(OH)2

h) 0.75 mol of Pb3(AsO4)2

7. Determine the number of moles in the following.

a) 35.00 g of Na

b) 15.0 g of Cs

c) 46.0 g of Cr

d) 28.0 g of Si

e) 77.0 g of CuO

f) 88 g of Al2(SO4)3

g) 50.0 g of NaNO3

h) 45.0 g of H3PO3

i) 24.0 g of Na3P

j) 0.95 g of O2

k) 125 g of CaCO3

l) 185.0 g of Ni(FO3)2

8. Determine the mass of the following.

a) 5.65 x 1024 molecules of CaBr2

b) 4.25 x 1023 molecules of N2

c) 9.10 x 1022 molecules of Ba(NO3)2

d) 3.01 x 1022 molecules of Ga2(CO3)3

9. Determine the number of molecules in each.

a) 77.0 g of CuO

b) 88.0 g of Al2(TeO4)3

c) 2.50 g of Ne

d) 71.0 g of MgS

10. Determine the number of atoms in each.

a) 85.0 g of Mn(OH)2

b) 125 g of CuSO4

c) 375 g of Sn(TeO4)2

d) 26.0 g of SO3

Answers: