Difference Between Molarity and Molality

A bulk of reactions in the laboratories are performed with different solutions. Thus it is essential to know how the amount of substance is said when it is present in the form of a solution.

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The amount of a given substance present in the given volume or the concentration of a given solution can be expressed in Molarity, Molality, Normality, etc.

Molarity vs Molality 

The difference between Molarity and Molality is that molarity focus on the number of mole of solute per litres of the given solution, while molality deals with the mole of solute per kg of the solvent. Molarity is represented by ‘M’ or Molar or mol/L, while Molality is represented by ‘m’ or Molal or mol/kg.

Molarity vs Molality

The molarity of a solution is dependent on the temperature and inversely changes with it. If the temperature is increased, the molarity decreases, while if the temperature is decreased, the molarity increases.

The value of molarity is affected by the change in pressure also. The value of molarity is not accurate or precise.

The molality of a solution is independent of the change in temperature and pressure. This is because the mass of the solute remains unaffected by any change in temperature and pressure.

The value of molality is quite accurate and precise. The molality of a solution is concerned with the mass of the solvent.

Comparison Table

Parameters of ComparisonMolarityMolality
DefinitionIt is defined as a mole of solute dissolved in a litre of the solution.It is defined as a mole of solute dissolved in 1 kg of the solution
SI Unit mol/Lmol/kg
Measurement It is measured by means of the volume of solution.It is measured by means of the mass of the solvent.
Temperature Affected by any changes in the temperatureDo not affect by any changes in the temperature.
Denoted Mm
AccuracyIt is inaccurate and is not reliable.It is precise and accurate.
PressureAffected by any change in pressureDo not affect by any change in pressure.

What is Molarity?

The number of moles of the given solute dissolved in a given per litre of the solution is known as Molarity. Molarity is represented by the symbol ‘M’. The formula for calculating molarity mathematically is given below –

M = No. of moles of a given solute/volume in 

litres

The standard unit of Molarity is moles/L, or Molar or M. Consider the example given below – 0.25 mol/L solution of sodium hydroxide means that 0.25 moles of sodium hydroxide have been dissolved in the 1 litre (or 1 cubic decimetre) of the solution.

As the volume of the given solution increases with an increase in temperature of the solution, then the Molarity of the solution also decreases along with it while the volume of the solution decreases with a decrease in temperature.

Thus the Molarity of the solution increases with a decrease in temperature.

For complex problems, the given equation M1V2 = M2V2 is used to find out the Molarity of the solution. For example – Dissolve 4 g of sodium hydroxide in 250 ml of water. What is the molarity of the above solution?

Solution – Mass of Sodium hydroxide = 4.0 g

Molar Mass of Sodium hydroxide, NaOH = 23 (Na) + 16 (O) + 1 (H) = 40.0 g/mol

Volume of water in litre = 250/1000 = 0.25 L

Number of moles = Mass of sodium hydroxide/Molar mass of sodium hydroxide

                                = 4/40 = 0.1 mol

Molarity = Number of moles of sodium hydroxide/Volume of solution in litre

               = 0.1/0.25 = 0.4 M

molarity

What is Molality?

The number of moles of the solute per kilogram of the solvent is known as Molality. It is denoted by the symbol ‘m’. The formula for calculating molality mathematically is given below –

Molality = Moles of given solute/Mass of given solvent in kg

The standard unit of measurement of Molality is moles/kg or Molal or m. Consider the example given below – 1.0 mol/kg solution of potassium chloride means that 1 mole (74.5 g) of potassium chloride is dissolved in 1 kg of solution.

It is not affected by the change in temperature since mass remains constant or unaffected by temperature.

Consider the following example for better understanding – When 2.5 g of ethanoic acid is dissolved in 75 g of benzene.

Solution – Molar mass of ethanoic acid = 60 g/mol

Moles of ethanoic acid = 2.5/60 = 0.0417 mol

Mass of benzene in kg = 7/1000 kg

The molality of ethanoic acid = Moles of ethanoic acid / Mass of benzene                                             

= 0.0417 × 1000 / 75 = 0.556 mol/kg 

molality

Main Differences Between Molarity and Molality

  1. The molarity of a solution can be defined as a mole of solute dissolved in a litre of the solution, while the Molality of a solution can be defined as a mole of solute dissolved in a kg of solution.
  2. The SI unit used to express molarity is ‘mol/L’, while for molality, it is ‘mol/kg’.
  3. The molarity of a solution can be measured on the basis of the volume of a given solution. On the other hand, the molality of a solution can be measured on the basis of the mass of the given solvent.
  4. Molality changes with the fluctuation in temperature, while the fluctuation in temperature does not affect the value of molality.
  5. The molality of a solution can also be denoted by ‘M’, while the molality of a solution is denoted by ‘m’.
  6. The value of molarity is not considered accurate and precise. On the other hand, the value of molality is more accurate and precise.
  7. The change in the pressure affects the value of molarity, but the value of molality remains unaffected by the change. 

References

  1. https://www.mdpi.com/2218-0532/77/4/817
  2. https://acp.copernicus.org/articles/18/11125/2018/
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