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Shrinithi Mahadevan

HS-PS1-5

Molality Study Guide

Molality can be understood as the total number of moles of a solute contained in a kilogram of a solvent. It is also termed as a molal concentration.

INTRODUCTION

What does one mean by molality? What is molality in chemistry? Molality is a characteristic of any solution, defined as the total number of moles of solute present per kilogram of solvent.

A solution primarily consists of two components- solvent and solute. There are various ways and calculations to express the concentration of solutions, like molarity, normality, molality, volume and weight percentage, and formality alongside part per million.

WHAT IS MOLARITY?

Molar concentration measures the chemical species concentration, particularly a solute in a solvent, in terms of the amount of substance per unit volume of solution.

                               M = n/v 

where M equals molar concentration, N is moles of solute, and v is liters of solution.

MOLALITY AND its FORMULA

The molality of a solution is often defined by 'm' and is the amount of solute present in moles, divided by the total mass in a kilogram of the solvent.

Molality Source

Compared to other molar concentrations or even mass concentrations, the solution preparation of a given molality is easier. Besides, in some weak aqueous solutions, the value of molality and molarity are almost similar. This is because 1 kg of water takes up 1 liter of volume at normal temperature, and the negligible amount of solute has significantly less effect on the overall volume.

Unlike molarity which completely depends on solution volume, molality is a intensive property and is only calculated as moles of a solute divided by the total kilograms of the solvent. Molarity varies greatly because of variation in volume due to external factors like temperature and pressure. When working with a wide temperature range, molality is taken as the standard calculation.

For example, we know that salt easily gets dissolved in water. Does that mean you cannot calculate the molality? If you only know the basics, including the mass of salt and water, the molality could be easily determined.

Molality Source

WHAT IS THE UNIT OF MOLALITY?

The SI unit of molality is expressed as mol/kg, or it can be termed as moles solute per kilogram of solvent.

HOW TO CALCULATE MOLALITY?

Calculating molality is quite simple, and one needs to figure out the mass of solute and the solvent in the solution. This property is intensive and is entirely independent of the measured amount. Molality, or the concentration, always remains constant.

Molality Source

CONCLUSION:

  1. Molality is a property of any solution, defined as the moles of any given solute present per kilogram of solvent.
  2. The SI unit of molality is expressed as mol/kg, or it can be termed as moles solute per kilogram of solvent.
  3. To calculate the molality, one needs to figure out the solute's mass and the solvent in the solution.

FAQs:

1. What is molality in chemistry?

Molality is a property of any solution, which can be termed the moles of any given solute present per kilogram of solvent.

2. What is an example of molality?

An example would be one molal NaOH stands for 1 mole of NaOH in 1kg water.

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SOURCES:

  1. Molality: https://byjus.com/chemistry/molality/. Accessed 28 Feb 2022.
  2. Molality: https://courses.lumenlearning.com/introchem/chapter/molality/. Accessed 28 Feb 2022.
  3. Molality: https://www.ck12.org/c/chemistry/molality/lesson/Molality-CHEM/. Accessed 28 Feb 2022.