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)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