How To Find Out The Molarity And Molality Of A Solution

Learn what is molarity and molality and how solute and solvent are important for calculating molarity and molality of solutions.

by Asma Rahil | Published July 16, 2021
Last Updated July 19, 2021
How To Find Out The Molarity And Molality Of A Solution

Table of contents

As we all know that a chemical solution comprises two main components; solute and solvent. In a chemical solution, the component present in the highest concentration is known as a solvent. On the other hand, the other components that are present in smaller concentrations are called solutes. Solutes could be solid, liquid, or gas.

However, to know about the exact concentration of a given solution, we use specific concentration units. Why cannot we use already available measurement units like volume or mass? Because these measurement units are too large for measuring tiny particles of solute and solvent.

Thus, to study the chemical solutions, we use a unit based on mole - MOLARITY & MOLALITY

To know about mole and calculations based on the mole, read our article How to find molar mass, formula mass, and percentage composition?

Molarity

Suppose you are provided with a solution whose total volume is one liter, and it possesses three moles of solute. We will say that the molarity of that solution is three.

Thus, molarity (also known as molar concentration) can be defined as the number of moles of solute present or dissolved in one liter of solution. Keep in mind that we are here considering the number of liters of total solution other than that of solvent. Mathematically, molarity can be represented such as

Molarity = number of moles of solute/liters of solution = mol/L

To represent the molarity, we use the unit moles/liter that is shown by the symbol "M." Thus, whenever we see a solution labeled as 2M NH4Cl, we will read it as two molar solutions of ammonium chloride.

How to calculate the molarity of a solution?

Suppose you are provided with a solution of 500ml, which possesses 42.23g of NH4Cl. To find the molarity of that solution, follow these steps.

  1. What you know

    2. The problem shows that the total mass of NH4Cl present in the solution is 42.23 g. Moreover, the total volume of the solution is also 500ml. From the compound formula, we can also measure the molar mass of NH4CL, which is 53.50 g/mol.

    To learn how to calculate molar mass from the formula of a compound, read our article on how to find out molar mass, formula mass, and percentage composition.

  2. Convert the units

    3. According to the formula of molarity (M=Moles/liters), we have to convert the volume of solution in liters and the mass of solute in moles such as

    • Change ml into liters

      • L = given mass in g/1000 = 500/0.5000
      500 ml = 0.5000 L

    • Change mass into moles

      • Number of moles = 1 mole of solute/molar mass × given mas
      Number of moles = 1 mol of NH4CL/53.50g NH4CL × 42.23g NH4CL = 0.7893 mol NH4CL

  3. Calculate the molarity

    4. Molarity = number of moles of solute/liters of solution
    Molarity = 0.7893moles/0.5000L = 1.579M

    Thus, the molarity of the given solution is 1.579M.

Molality

The Molality of a solution is known as its molal concentration. We can define the molality of a solution as

"The number of moles of solute present in the total mass of solvent"

Remember that here, unlike molarity, we consider only the mass of solvent other than the entire solution. Moreover, we are taking the mass of the solvent in kilograms other than liters. Mathematically, molality can be represented as

Molality = number of moles of solute/mass of solvent in kilograms = moles/kg.

The unit of molality is moles/kg, which is represented by m.

How to Find the Molality of a Solution?

Suppose you are asked to find out the molality of a solution that possesses 225g of benzene, in which 0.5g of toluene has been dissolved. To find the morality of this solution, follow these steps.

  1. What you know

    2. The problem showing that you have
    Solute(toluene) = 0.5g
    Solvent(benzene) = 225g
    Molar mass of toluene (C7H8) = 92 g/mol

    To get the most exact and precise molar mass, you need to use online molar mass calculator.

  2. Convert the units

    3. According to the formula (moles/kg), we have to convert the mass of solute into the number of moles and mass of solvent from grams to kilograms.

    • Change mass into moles

      • Moles of toluene = Given mass of toluene/molar mass of toluene Moles of toluene = 0.5/92 = 0.054 moles

    • Change grams into kilograms

      • Kilograms = given mass in grams/1000 = 225/1000 = 0.225 kg

  3. Calculate the molality

    4. Molality = number of moles of solute/ mass of solvent in kg
    M = 0.054 moles/ 0.225 kg = 0.24m

    Thus, the molality of the given solution is 0.24m.

Bottom Line

Molarity and morality are often confused with each other. However, there are some major differences between both these parameters, such as

  • In molarity, we take the total volume of solution(in liters) into consideration. On the other hand, we only consider the mass(in grams) of solvent while calculating molality.
  • The unit of molarity is moles/liters, which is shown by "M." On the other hand, the unit of molality is moles/kg, which is shown by "m."
  • Molarity depends on temperature and pressure, while molality remains constant even in T and P changing conditions.