142.Concentration units.

In analytical chemistry one often has to deal with solutions and thus it is very important to know parameters used to define concentration.

A solvent is a substance that dissolves solute in it to form a solution.
e.g.– Sugar solution is sugar dissolved in water.

Concentration is a general measurement term used to describe the amount of solute present in a known amount of solution.

Suppose we take a glass of water and add 1 tablespoon of sugar and then we take another glass and add two tablespoons of sugar to it. Which of the two solutions is more concentrated? Using common-sense we can be sure that the second solution is more concentrated as it contains more sugar. However, there are concentration terms that help us describe the concentration of solutions scientifically. Let us discuss the various concentration terms used in chemistry –

1. Molarity

It indicates the number of moles of solute present per liter of solution.

∴ M = n/V

e.g. – If we dissolve 1 mole of sugar in 1 liter of water, we get 1 molar sugar solution.

The SI unit of molarity is mol/lit.

 

2. Molality

This term indicates the number of moles of solute per kilogram of solvent.

∴m=n/M

The SI unit of molality is mol/kg.

3. Formality

Just like molarity, formality also indicates the number of moles of solute per liter of solution. The difference between these two terms is that formality term is used for substances that ionize in solutions.

Thus, for a substance that does not ionize in solution →
Molarity = Formality. 
However, for a substance that ionizes → Molarity ≠ Formality.

Thus, molarity(M) is the number of moles of solute per liter of solution, and formality(F) is the number of moles of solute, regardless of chemical form, per liter of solution.

For example, when we dissolve 0.1 mol of NaCl in water, it dissociates to give 0.1 mol of Na⁺ and Cl^– ions .

Note that undissociated NaCl does not exist on the solution and thus, the molarity of this solution will be zero.

The formality of the solution will be 0.1 F.

The formula weight can be calculated by adding up all the atomic masses of atoms in the formula for a given compound.

e.g.- The formula weight of potassium permanganate (KMnO₄) will be –

Atomic weight of potassium + atomic weight of Manganese + 4 × atomic weight of oxygen
= 39 + 55 + 4(16)
= 158.
This compound will dissociate in water to give K⁺ and Mn0₄^– ions –

Formality is a less used term and one encounters this term very rarely.

In the next post we will discuss some more concentration units. Till then,

Be a perpetual student of life and keep learning…

Good Day!

Image source –

1. https://city-press.news24.com/Special-Report/Budget-2017/health-industry-welcomes-sugar-tax-but-reckons-it-should-have-been-higher-20170222
3. https://www.instructables.com/id/How-to-fill-a-cup-with-water/