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:
| Element | Mass (g) |
|---|---|
| Na | 23 |
| Molecule | Calculation | Mass (amu) | Mass (g) |
|---|---|---|---|
| N₂ | 14 × 2 | 28 | 28 |
| Molecule | Calculation | Mass (amu) | Mass (g) |
|---|---|---|---|
| C₁₂H₂₂O₁₁ | (12 × 12) + (1 × 22) + (16 × 11) | 144 + 22 + 176 | 342 |
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 (0,) during thunder storms. Calculate the mass of ozone if 9.05 moles of ozone is 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.
9.05 moles of O, —> ?g of
Solution:
| Moles of O₂ | Calculation | Mass (g) |
|---|---|---|
| 1 mole | 16 × 3 | 48 g |
| 9.05 moles | 48 × 9.05 | 434.4 g |
Problem solving strategy:
Carbon dioxide is a molecular substance. Determine its molar mass and use it to convert moles to mass in grams.
0.25 molers of CO 2 ---> ? g of CO 2
Solution:
| Moles of CO₂ | Calculation | Mass (g) |
|---|---|---|
| 1 mole | 12 + 16 × 2 | 44 g |
| 0.25 moles | 44 × 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.
mass ---> ? moles
Solution:
| Substance | Calculation | Mass / Moles |
|---|---|---|
| H₂ | 1.008 × 2 | 2.016 g |
| 1 g of H₂ | 1 / 2.016 | 0.496 moles |
| 5 g of H₂ | (1 / 2.016) × 5 | 2.48 moles |
| H₂O | 2 × 1.008 + 16 | 18.016 g |
| 1 g of H₂O | 1 / 18.016 | 0.0555 moles |
| 100 g of H₂O | (1 / 18.016) × 100 | 5.55 moles |
1. The molecular formula of a compound used for bleaching hair is H,O,. Calculate (a) Mass of this compound that would contain 2.5 moles. (b) No. of moles of this compound that would exactly weigh 30g.
2. Aspoon of table salt, NaCl contains 12.5grams of this salt. Calculate the number of moles it contains.
3. Before the digestive systems X-rayed, people are required to swallow suspensions of barium sulphate BaSO, Calculate mass of one mole of BaSO,.
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. 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. At high temperature hydrogen sulphide (H₂S) gas given off by a volcano is oxidized by air to sulphur dioxide (SO₂). Sulphur dioxide reacts with water to form acid rain. How many molecules are there in 0.25 moles of SO₂
Remember that CH₄ is a molecular compound, thus 1 mole of methane will have 6.022 x 1023 molecules. Similarly, SO₂ is a molecular compound, its one mole will also have 6.022 x 1023 molecules.
Titanium is corrosion resistant metal that is used in rockets, aircrafts and jet engines. Calculate the number of moles of this metal in a sample containing 3.011 x 1023 Ti-atoms.
Remember that 1 mole of an element contains 6.022 x 1023 atoms.
Thus, 6.022 x 1023 atoms = 1 mole
3.011 x 1023 atoms = ? moles
6.022 x 1023 Ti atoms = 1 mole of Ti
1 Ti atom = 1/6.022 x 1023 moles of Ti
3.011 x 1023 Ti atoms = (1/6.022 x 1023) × 3.011 x 1023 moles of Ti
= 0.5 moles of Ti
Formaldehyde is used to preserve dead animals. Its molecular formula is CH₂O. Calculate the number of moles that would contain 3.011 x 1023 molecules of this compound.
Remember that 1 mole of any compound contains 6.022 x 1023 molecules.
Thus, 6.022 x 1023 molecules = 1 mole of compound
3.011 x 1023 molecules = ? moles
6.022 x 1023 molecules = 1 mole of formaldehyde
1 molecule = 1/6.022 x 1023 moles of formaldehyde
3.011 x 1023 molecules = (1/6.022 x 1023) × 3.011 x 1023 moles of formaldehyde
= 0.5 moles of formaldehyde
1. Aspirin is a compound that contains carbon, hydrogen and oxygen. It is used as a painkiller. An aspirin tablet contains 1.25 x 10 molecules. How many moles of this compound are present in the tablet?
2. A method used to prevent rusting in ships and underground pipelines involves connecting the iron to a block of a more active metal such as magnesium. This method is called cathodic protection. How many moles of magnesium are present in 1 billion (1 x 10") atoms of magnesium.