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Explain the term mole concept.
As we know that the atoms and molecules are very small in size, and the numbers of a small substance are very large, handling such large amounts can only be done by providing a unit of convenient magnitude. So, we use the idea of the mole to count those entities at a microscopic level. In the SI system, the mole was used as the seventh base quantity for the amount of substance and is denoted by ‘mol’.
What are the various formulas used in the mole concept?
Mass percentage of an element: –
For calculating mass% of an element, we use the formula:
Mass% of an element = mass of the element in the compound × 100/ molar mass of a compound
For understanding it better, let’s take an example,
Water (H2O) contains both hydrogen and oxygen, so the percentage of both these can be calculated by using the above formula
Mass% = mass of the element in the compound × 100/ molar mass of a compound
The molar mass of water = 18.02g
Mass% of hydrogen = 2×1.008/18.02 ×100
= 11.18%
Mass% of oxygen = 16.00/18.02 × 100
= 88.79%
Mass percent: –
Mass percent = mass of solute/mass of solution × 100
For better understanding, let’s look at the following question,
A solution was prepared by adding 2g of substance A to 18g of water. Now, calculate the mass percent of solute.
Solution:
Mass percent of A = mass of A/ mass of solution × 100
= 2g/ 2g of A+ 18g of water × 100
= 2g/20g × 100
= 10%
Mole fraction: – the ratio of the number of moles of a particular component to the number of moles of the solution is called a mole fraction.
If a substance A dissolves in substance B and their number of moles are nA and nB, respectively, then the mole fraction of A and B is given as:
Mole fraction of A = no. of moles of A/ no. of moles of solutions
= nA / nA + nB
Mole fraction of B = no. of moles of B/ no. of moles of solutions
= nB / nA + nB
Molarity is defined as the number of moles of the solute in 1 litre of solution.
Molarity (M) = No. of moles of solute/ volume of solution in litres
Suppose we have 1 M of a substance; for example, let’s take NaOH, and we want 0.2 M solution from it.
Here, 1 NaOH means 1 mol of NaOH present in 1 litre of solution. For 0.2 M solution, we need 0.2 moles of NaOH dissolved in 1 litre of solution.
So, we have to take that volume of 1M NaOH solution, which contains 0.2 mol of NaOH, and dilute it in 1 litre of water.
Now, if 1 mol is present in 1L or 1000 mL solution
Then, 0.2 mol is present in
1000 mL / 1 mol × 0.2 mol solution
= 200 mL solution. This means that 200 mL of 1 M NaOH is taken, and enough water is added to make it 1 litre.
Note: the molarity of a solution depends upon the temperature because the volume of the solution is temperature-dependent.
Molality: – the number of moles of solute present in 1 kg of solvent. It is denoted by m.
Molality (m) = no.of moles of solute/ mass of solvent in kg
Let’s do one numerical
The density of the 3 M solution of NaCl is 1.25 g mL-1. Calculate the molality of the solution.
Solution: –
M = 3 mol L-1
Mass of NaCl in 1 L solution = 3 × 58.5
= 175.5 g
Mass of 1 L solution = 1000 × 1.25
= 1250 g (since density = 1.25 g mL-1)
Mass of water in solution = 1250 – 75.5
= 1074.5 g
Molality (m) = no. of moles of solute/ mass of solvent in kg
= 3 mol / 1.0745 kg
= 2.79 m
Conclusion:
The study of chemistry is very important because its domain encompasses every sphere of life. It is the study of the properties and structure of substances and their changes. When the properties of a substance are studied, then the entities involved in a substance, i.e., atoms, molecules and other microscopic elements, measurement is inherent. That’s why measuring the microscopic entities concept of the mole was used. And for calculating the amount of substance present in a given volume can be measured in several ways, i.e., mass percent, mole fraction, molarity and molality.
