3.2.4 Number of Mole and Mass of a Substance
Molar Mass
- The mass of one mole of atoms is its relative atomic mass in grams, and is called a molar mass.
- A Molar mass is the mass of a substance containing the Avogadro Constant of particles.
Element | Relative atomic mass | Mass of 1 mol of particle | Molar Mass |
Hydrogen | 1 | 1 g | 1 g mol-1 |
Carbon | 12 | 12 g | 12 g mol-1 |
Oxygen | 16 | 16 g | 16 g mol-1 |
Copper | 64 | 27 g | 27 g mol-1 |
Iron | 56 | 56 g | 56 g mol-1 |
Example
- The relative atomic mass of copper is 64, therefore the mass of 1 mole copper is 64g
- The RAM of sodium is 23, therefore the mass of 2 mole sodium is 46g
- The RAM of nitrogen is 14, therefore the mass of 2.5 mole l nitrogen is 35g
- The RAM of calcium is 20, therefore the mass of 0.3 mole calcium is 6g
Conclusion
Note:
The relationship between the mass of a substance and the number of mole of the particles in the substance can be summarised by using the following equation:
n= m Molar Mass
If you are given the mass of substance and asked to find the number of mole of the substance (or vice versa), the problem can be solved by using this equation.
Example:
Find the number of mol of atoms in 4.6g sodium [Relative atomic mass: Na=23]
Answer:
Number of mole, n =4.6/23 = 0.2 mol]
Example
How many moles of each substance are there in 191 g NaOH [Relative atomic mass: Na=23, O=16, H=1]
Answer:
Relative Formula Mass of NaOH = 23 + 16 + 1 = 40
Number of mole of 191g NaOH,
[n = 191/40 = 4.775 mol]
Question
What is the mass of 7.12 mol NaI. [Relative atomic gas: Iodine = 131; Sodium = 23]
Answer:
The relative formula mass of NaI = 23 + 131 = 154.
The mass of 7.12 mol NaI
= Number of mole x Relative Foemula Mass of NaI
= 7.12 x 154
= 1096.48g