In this section, you will learn about the chemical calculations based on the concept of mole and Avogadro's number.
Calculate the molar masses of:
(a) Na
(b) Nitrogen
(c) Sucrose C12H22O11
Problem solving strategy:
If an element is a metal then its molar mass is its atomic mass expressed in grams (gram atomic mass). If an element exists as a molecule, its molar mass is its molecular mass expressed in grams (gram molecular mass).
Solution:
a) 1 mole of Na = 23 g
b) Nitrogen occurs as diatomic molecules. Molecular mass of N2 = 14 x 2 = 28 amu = 28 g
c) Sucrose (C12H22O11): Molecular mass = 12 x 12 + 1 x 22 + 16 x 11 = 144 + 22 + 176 = 342 g
1. Calculate the mass of one mole of:
(a) Copper
(b) Iodine
(c) Potassium
(d) Oxygen
2. Differentiate between gram formula mass and gram molecular mass.
Oxygen is converted to ozone (O3) during thunderstorms. Calculate the mass of ozone if 9.05 moles of ozone are formed in a storm.
Problem solving strategy:
Ozone is a molecular substance. Determine its molar mass and use it to convert moles to mass in grams.
Solution:
Molar mass of O3 = 16 x 3 = 48 g
Mass of 9.05 moles of O3 = 48 g x 9.05 = 434.4 g
If 0.25 moles of CO2 are formed, what mass of CO2 is produced?
Problem solving strategy:
Carbon dioxide is a molecular substance. Determine its molar mass and use it to convert moles to mass in grams.
Solution:
Molar mass of CO2 = 12 + 16 x 2 = 44 g
Mass of 0.25 moles of CO2 = 44 g x 0.25 = 11 g
How many moles of each of the following substances are present?
(a) A balloon filled with 5 g of hydrogen.
(b) A block of ice that weighs 100 g.
Problem solving strategy:
Hydrogen and ice are molecular substances. Determine their molar masses. Use the molar mass of each to convert the masses in grams to moles.
Solution:
a) Molar mass of H2 = 2 x 1.008 = 2.016 g/mol
Moles of 5 g of H2 = 5 g / 2.016 g/mol = 2.48 moles
b) Molar mass of H2O = 2 x 1.008 + 16 = 18.016 g/mol
Moles of 100 g of H2O = 100 g / 18.016 g/mol = 5.55 moles
1. The molecular formula of a compound used for bleaching hair is H2O2. Calculate:
(a) Mass of this compound that would contain 2.5 moles.
(b) Number of moles of this compound that would exactly weigh 30 g.
2. A spoon of table salt, NaCl, contains 12.5 grams of this salt. Calculate the number of moles it contains.
3. Before the digestive system is X-rayed, people are required to swallow suspensions of barium sulfate (BaSO4). Calculate the mass of one mole of BaSO4.
1. Zn is a silvery metal which is used to galvanize steel to prevent corrosion. How many atoms are there in 1.25 moles of Zn?
2. A thin foil of aluminium (Al) is used as a wrapper in food industries. How many atoms are present in a foil that contains 0.2 moles of aluminium?
Problem solving strategy:
Remember that symbols Zn and Al stand for one mole of Zn and Al atoms respectively.
Solution:
1. 1 mole of Zn contains 6.022 x 1023 Zn atoms
1.25 moles of Zn contain = 6.022 x 1023 x 1.25 = 7.53 x 1023 Zn atoms
2. 1 mole of Al contains 6.022 x 1023 Al atoms
0.2 moles of Al contain = 6.022 x 1023 x 0.2 = 1.2044 x 1023 Al atoms
1. Methane (CH4) is the major component of natural gas. How many molecules are present in 0.5 moles of a pure sample of methane?
2. What is the number of molecules in 1.6 moles of H2O2?
Problem solving strategy:
Convert moles of substance to number of molecules using Avogadro's number.
Solution:
1. 1 mole of CH4 contains 6.022 x 1023 molecules
0.5 moles of CH4 contains = 6.022 x 1023 x 0.5 = 3.011 x 1023 molecules
2. 1.6 moles of H2O2 contains = 6.022 x 1023 x 1.6 = 9.635 x 1023 molecules
1. Calculate the number of molecules in the following:
(a) 0.5 moles of CH4
(b) 2.5 moles of NH3
2. Calculate the number of moles in:
(a) 1.2 x 1023 molecules of CH4
(b) 5.1 x 1022 molecules of H2SO4