Molarity vs. Molality: Which One to Prefer?

Molarity and molality both measure the concentration of solutions. Molarity (M) is the measure of moles of solute per liter of solution. Whereas molality (m) is the measure of moles of solute per kilogram of solvent.

Molarity and molality are the two most common units for the determination of the concentration of solutions. Other units include normality, mass percent, mole fraction, formality, volume percent, etc.

Molarity vs Molality

Molarity Molality
Molarity of solution is the ratio of moles of solute to the volume of solution in liters Molality of solution is the ratio of moles of solute to the mass of solvent in kilograms
It is denoted by M It is denoted by m
M = moles of solute/ liters of solution m = moles of solute/kilograms of solvent
Molarity depends on the volume of whole solution Molality depends only on the mass of solvent
It is dependent on pressure and temperature It is not effected by changes in pressure and temperature
The unit of molarity is mol/L The unit of molality is mol/kg

Dependence on temperature and pressure

Unlike molality, molarity depends on the volume of the solution. Since the volume of a solution changes with the change in temperature and pressure, molarity also varies with the changing temperature and pressure (atmospheric pressure).

Formula of Molarity M

Molality is an intensive property of solutions. It only depends on the mass of solvent and hence, it does not change with temperature and pressure.

Formula of Molality m

When to use molarity and molality

Molality is generally preferred, especially when the quantities are expected to change with temperature and pressure. For example, when working on boiling points, melting points, and colligative properties of solutions (elevation of boiling point, depression of freezing point), etc.

Molarity is mostly used in labs for the calculation of the concentrations of solutions because molar concentrations are easier to work with. Molarity is also important to figure out whether a solution is dilute or concentrated.

Please note that the difference between molarity and molality for aqueous solutions (where water is solvent) near room temperature is very small and it won’t really matter whether a molar or molal concentration is used. This is because around room temperature, water has a density of 1.0 kg/L. This means that, for standard conditions, the “per Litre” of molarity is equivalent to the “per kilogram” of molality.

Related topics

Key differences between molarity and molality

Molarity vs Molality which One to Prefer Key takeaways Difference table

Concepts berg

How to calculate normality from molarity?

When the molarity of acid or base solution is known, multiply it with the number of hydrogen ions in acid or hydroxyl ions in base to get normality.

For example

  1. 2 molar HCl will have a normality of 2N (2M x 2 hydrogen ions per HCl molecule)
  2. 3M H2SO4 solution will have a normality of 6N (3M x 2 hydrogen ions per sulphuric acid molecule).
  3. Similarly, A 5M H3PO4 solution will have a normality of 15N, etc.

How do you convert molarity to molality?

Molality is equal to molarity divided by the solution’s density at that temperature. (m = M/d)

Which is better molality, or molarity?

Both molality and molarity are concentration units. Molarity is generally preferred, whereas molality is used when temperature and pressure changes are problematic.

What’s the purpose of using molarity vs molality and when is it appropriate to choose between the two?

Molar calculations are way easier and therefore, often preferred in labs. However, molar solutions being dependent on temperature and pressure are avoided when studying the changes due to vapor pressure and temperature, etc and hence molality is used.

What are the molality and molarity of water?

The molarity of water is 55.348 Molar and the molal concentration of water is 55.510 molal.

What depends on temperature, molality, or molarity?

Molarity depends on temperature since it is dependent on the volume of solution which is affected by temperature changes.

Which is higher in concentration 1 molar or 1 molal solution?

1 molar solution is more concentrated as it contains 1 mole of solute in 1L of solution (both solute and solvent make up 1000mL). Whereas 1 molal solution contains 1 mole of solute in 1000g of solvent. (1 mole of solute + 1000g of solvent). Hence, the molar solution will contain less amount of solvent than the molal solution, provided that the moles of solute are the same.

References

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