0Expressing the concentration of a solution:-
Before we start with the concentration, we should understand the calculation of moles.
A number of moles =mass of solute /Molecular mass of the substance.
Example: Number of moles in 80g of NaCl=80/40=2(molecular mass of NaCl=40)
Molarity
Now let us see the calculation of molarity of a solution which is already discussed in class11.
Molarity=No of moles of solute/volume of solution in litres
Molarity=W2X1000/M2XV(W2 =mass of solute, M2=molecular mass of solute, V=volume of solution)
Example-1
Calculate the molarity of a solution containing 8g of NaCl in 500ml solution.
Molarity of the solution=8X1000/40X500=0.4moles/L
Molality
It is the number of moles of solute in one kg of the solvent.
Molality=W2X1000/M2XW1(W1=mass of solvent)
Example-2
Calculate the molarity of a solution containing 18g of glucose in 700g of water.
Molality of the solution=18x1000/180x700=0.14moles/kg
Molarity and molality are always confusing for the students. It is better to study the formulae for the calculation.
Q1.Molarity depends on temperature, whereas molality is independent. Why?
A. Molarity depends on volume which is affected by temperature changes. As a result, molarity varies. Molality depends on mass which is not affected by temperature.
Q2.One molar solution or one molal solution which is more concentrated?
One molar solution is more concentrated because it contains less water than one molal solution.
