Introduction
In chemistry and biochemistry, concentration is an important concept that describes how much solute is present in a given volume of solution. One of the most common ways to express concentration is using molarity. Molarity is used in many laboratory experiments for preparing chemical solutions in precise quantities.
Definition of Molarity
Molarity (M) is defined as the number of moles of solute dissolved in one liter (1,000 milliliters) of solution.
Formula:
Molarity (M) = Moles of solute / Volume of solution in liters
Molarity is expressed in units of moles per liter (mol/L or M).
Understanding Moles
One mole of a substance is equal to its molecular weight in grams. The molecular weight (molar mass) of a compound is calculated by adding the atomic weights of all the atoms in the compound.
For NaOH (Sodium Hydroxide):
- Sodium (Na) = 23 g/mol
- Oxygen (O) = 16 g/mol
- Hydrogen (H) = 1 g/mol
Total = 23 + 16 + 1 = 40 g/mol
How to Prepare 100 mL of 0.5 M NaOH Solution
To prepare this solution, follow these steps:
Step 1: Use the Molarity Formula
We are asked to prepare 100 mL of 0.5 M NaOH.
First, convert 100 mL to liters:
100 mL = 0.1 L
Use the formula:
Moles = Molarity × Volume (in liters)
Moles of NaOH = 0.5 M × 0.1 L = 0.05 moles
Step 2: Convert Moles to Grams
Mass = Moles × Molar Mass
Mass = 0.05 × 40 g/mol = 2 grams
Step 3: Procedure to Prepare the Solution
- Weigh exactly 2 grams of solid NaOH using an electronic balance.
- Take a clean 100 mL volumetric flask.
- Add about 50 mL of distilled water to the flask.
- Dissolve the 2 grams of NaOH in the water. Use a glass rod or shake gently to help it dissolve completely.
- After the NaOH is fully dissolved, add more distilled water slowly until the total volume reaches exactly 100 mL mark on the flask.
- Cap the flask and shake gently to ensure uniform mixing.
Precautions
- NaOH is corrosive. Always wear gloves and safety goggles while handling it.
- Always add NaOH to water, never the reverse, to avoid splashing due to heat generation.
- Make sure to use distilled water to avoid contamination.
Conclusion
Molarity is a useful concept in solution preparation. To prepare a 0.5 M NaOH solution in 100 mL of water, you need to dissolve 2 grams of NaOH and dilute it up to 100 mL with distilled water. This method is widely used in biochemistry and chemical laboratories for preparing accurate solutions required in experiments.