#### The Mole - Sample Problems

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)→  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