The Mole Concept

• the extreme smallness of atoms and molecules makes it necessary to define a very large “standard sample”

• it is referred to as the MOLE

• it’s the SI base unit for amount of chemical substance

• one mole consists of 6.02 x 10²³ atoms or molecules; it is known as Avogadro’s Number

• the mass of one mole of an element is the same as the atomic mass of the element

for example:
one mole of bromine, atomic mass 79.90 amu, has a mass of 79.90 g; this is called the molar mass

the molar mass of chromium, Cr, is 52.00 g/mol.
 

types of calculations:

(i) mass of 1.50 moles of chromium

(ii) # of moles in 5.00g of O₂

(iii) determining the molar mass of a compound; what is the molar mass of CuSO₄?

(iv) # of moles given a particular mass of a compound; how many moles is 56.00 g of NaCl?

(v) mass of a compound given a particular # of moles; determine the mass of 0.1 moles of H₂SO₄.

Examples:

1. How many grams of sulfur are present in 0.150 mol of Au?

solution:

1 mole of Au → 197.0 g

0.150 mol of Au→ x g

x = 197.0 g x 0.150 moles = 29.6 g Au

              1 mole

2. How many atoms are in a sample of uranium with a mass of 1.00 x 10^–6 g ?

 solution:

 1 mol U → 238.0 g U

1 mol U → 2.03 x 1023 atoms

 
therefore:

238.0 g U → 2.03 x 1023 atoms

1.00 x 10–6 g U → x atoms

x = (2.03 x 1023 atoms)( 1.00 x 10–6 g) = 2.53 x 1015 atoms

                    238.0 g

 3. How many grams is 0.115 mol of Ca₃(PO₄)₂ ?

Solution:

1 mol of   Ca₃(PO₄)₂ →  310.18 g

0.115 mol  → x g

x = 310.18 g x 0.115 mol = 35.7 g Ca₃(PO₄)₂

             1 mol

4. How many grams of Cl are needed to combine with 24.4 g of Si to make silicon tetrachloride, SiCl₄?

Solution:

The balanced equation for the reaction must be written

            Si(s) + 2Cl₂(g) → SiCl₄(s)

From the equation we see that:

1 mol Si reacts with 2 mol of Cl₂                                1 mol Si = 28.09 g

                                                                                    x mol Si = 24.4 g

0.869 mol Si reacts with x mol Cl₂

                                                                                    x = 1 mol x 24.4 g = 0.869 mol Si

x mol Cl2  = 2 mol Cl₂ x 0.869 mol Si                                     24.4 g

                             1 mol Si                                          use this value in your calculations

                = 1.74 mol

1 mol Cl₂ = 70.90 g

1.74 mol Cl₂ = x g

x g Cl₂ = 70.90 g x 1.74 mol

                         1 mol

            = 123.366 g = 123 g Cl₂

Back to the top
Previous  Topic: Types of Reactions
Other: Activity Series of Metals