Molarity Formula

If you have recently taken up a lab course in basic chemistry, one of the first things that will bewilder you is molarity and why is it so important in applied chemistry. Going through this Buzzle article will hopefully clear your doubts regarding the concept of molarity (not to be confused with morality, an entirely different concern!).

What is Molarity?

In chemistry, molarity formula is something that you must know if you intend to undertake any experiments (and get good grades!). To understand that, you need to know what molarity means. Molarity, also known as molar concentration, is a measure of solute concentration in any solution. That is, it measures how much of a substance is dissolved in any solvent.

Solute is the substance that is mixed in a solvent. For example, consider a mixture of salt mixed in water. Here salt is the solute and water is the solvent and mixed together, they make a solution. To put it bluntly, molarity of salt water will be a measure of how much of salt has been mixed in a unit liter or a measure of salt concentration in water. However 'amount of salt' or solute is still vague. The amount of solute needs to be quantified in the definition of molarity as otherwise it would not be useful. That's why, molarity is defined in the following manner:

"Molarity is a measure of the concentration of a solute in a solution and is quantified as the number of moles of that substance, per unit volume of that solution."

To completely understand what the above definition means, one must know what the chemistry term - 'mole' of a substance is. For those of you who are not familiar with it, let me explain. A mole of a substance is its molecular weight expressed in grams. For example, the molecular weight of Oxygen is 16 and therefore one mole of oxygen is 16 gm. A mole of any substance contains exactly 6.023 x 10²³ (Avogadro's Number) molecules. To know the number of moles of any quantity of substance, you must divide the total weight by the weight of one mole.

Moles of a Substance = (Total Mass of the Substance)/(Gram Mole of that Substance)

The unit of molarity is mol/liter. Now that you have a rough idea of what molarity means, let me present the molarity formula in the next section. Know how to calculate molar mass.

Standard Molarity Formula

Molarity is denoted by the symbol 'M' when talking in quantitative terms and by 'c' when denoting it as a concept. Here are the standard molarity formulas that provide various ways using which you could calculate the molarity of a substance.

Molarity = n / V = N / (NA x V)

where,

• n = Number of moles of the substance
• V = Volume of the solvent (in liters)
• N = Number of molecules of substance present in the solution
• N_A = Avogadro number

So a solution of 1 M or 1 molar concentration has 1 mole of that substance per liter (1 mol/liter). In SI units, molarity may also be measured as 'moles per cubic meter (mol/m³)'. Normally molarity can be measured in 'Moles per decimeter cube (dm³)' as decimeter is a more commensurate unit.

Ergo, to calculate the molarity of a substance one must know the molecular weight of a substance, weight of the substance (from which number of moles can be calculated) and the volume of the solvent. Using this formula, solutions of definite concentrations can be created by chemists and they can quantitatively analyze results.

An Example

Problem: Find the molarity of 175.32 gm of Sodium Chloride (NaCl) dissolved in 2 liters of water (Given: Molar Mass of NaCl is 58.44 g/mol).

Solution: Firstly, we must compute the number of NaCl moles. Moles of NaCl = Total Mass of NaCl/Molar Mass = 175.32/58.44 = 3 Moles Now we can directly compute the molarity, using the above formula. Molarity of NaCl = number of Moles (n)/Total Solvent Volume (V) = 3 Moles/2 liters = 1.5 mol/liter

Hope, this article has cleared your doubts regarding molarity formula and how to use it. Chemistry is a practical science and quantifying the amounts of reactants and products or their concentrations is absolutely important, if experimental results are to make sense! That's why knowing the molarity formula and its use is an essential part of any basic course in chemistry.