Short Answer

Molarity is calculated by dividing the number of moles of solute by the total volume of solution in litres. The formula used is:
Molarity (M) = moles of solute ÷ litres of solution.

To calculate molarity correctly, you must first convert the solute amount into moles and measure the final solution volume in litres. This calculation helps chemists prepare solutions with accurate concentrations and perform chemical reactions with precise measurements.

Detailed Explanation

Calculation of Molarity

Molarity is a very important concept in chemistry because it shows how concentrated a solution is. It tells us how many moles of a solute are present in one litre of solution. To calculate molarity, we need to understand both the solute amount and the solution volume. The formula is simple, but applying it accurately requires clear steps and understanding of mole concepts.

1. Formula for calculating molarity

The formula for molarity is:

M = moles of solute ÷ litres of solution

This expression shows that molarity depends on two values:

• The amount of solute, expressed in moles
• The final volume of the entire solution, expressed in litres

If either value changes, the molarity also changes.

2. Steps to calculate molarity

To calculate molarity, follow these steps:

Step 1: Convert solute mass into moles

If the amount of solute is given in grams, convert it to moles using:

moles = mass ÷ molar mass

Example:
If you have 58.5 g of NaCl and its molar mass is 58.5 g/mol, then:
moles = 58.5 ÷ 58.5 = 1 mole

Step 2: Measure the total volume of solution in litres

Molarity is always based on litres of solution, not just the amount of solvent.
For example:

• 250 mL = 0.250 L
• 100 mL = 0.100 L
• 1 L = 1.0 L

The volume must be the final volume after dissolving the solute.

Step 3: Apply the molarity formula

After finding moles and volume, plug the values into:

M = moles ÷ litres

Example:
If 1 mole of solute is dissolved in 0.5 L of solution:
M = 1 ÷ 0.5 = 2 M

This means the solution has a concentration of 2 moles per litre.

3. Example problems

Example 1: Simple calculation

How many moles are in 2 litres of a 1.5 M solution?

M = n ÷ V
1.5 = n ÷ 2
n = 3 moles

Example 2: Mass to molarity

Dissolve 20 g of KOH (molar mass = 56 g/mol) in 0.5 L of solution.

Step 1: moles = 20 ÷ 56 ≈ 0.357 mol
Step 2: volume = 0.5 L
Step 3: M = 0.357 ÷ 0.5 ≈ 0.714 M

4. Why molarity calculation is important

Calculating molarity is crucial because:

1. It helps prepare accurate solutions

Chemists must know exactly how concentrated their solutions are, especially in titrations.

1. It allows stoichiometric calculations

Reactions in solutions need accurate mole amounts.
Example: Acid–base reactions rely on precise molarity.

1. It standardises laboratory measurements

Molarity is a universal unit that helps scientists work consistently.

1. It helps dilute or concentrate solutions

Using the formula M₁V₁ = M₂V₂, chemists adjust solution strengths.

1. It is used in research and industry

Pharmaceuticals, food processing, and water treatment use molarity to measure chemical concentrations.

5. Important points to remember during molarity calculation

• Always convert volume into litres.
• Always convert mass into moles.
• Use the total solution volume, not the solvent volume alone.
• Temperature affects volume, so molarity depends on temperature.
• Use correct molar mass of the solute.

These points ensure accurate calculations.

6. Temperature effect on molarity

Because molarity is based on volume, and volume changes with temperature, molarity is temperature-dependent. For example:

• When temperature increases → solution expands → volume increases → molarity decreases
• When temperature decreases → solution contracts → volume decreases → molarity increases

That is why molarity is always given at a specific temperature.

7. Molarity vs. other units of concentration

Molarity is often compared with:

• Molality (mol/kg of solvent)
• Normality (equivalents/L)
• Mass percent
• Volume percent

Among these, molarity is most commonly used for solution reactions because it is directly linked to moles.

Conclusion

Molarity is calculated by dividing the moles of solute by the total litres of solution. It provides a clear measure of how concentrated a solution is and is widely used in laboratory work, stoichiometry, titrations, and industrial processes. Understanding how to calculate molarity ensures accurate chemical preparation, safe experimentation, and reliable scientific results.