Sodium hydroxide, commonly known as lye or caustic soda, is a powerful and highly reactive inorganic compound used in various scientific and industrial processes. Preparing a 1 Molar (1M) solution involves dissolving a specific quantity of solid pellets into water to achieve a precise concentration. Because sodium hydroxide is extremely corrosive, a meticulous and controlled approach is necessary to ensure both accuracy and safety during its preparation.
Understanding Molarity and Initial Calculation
Molarity is the chemical measure of concentration, defined as the number of moles of a solute dissolved per liter of the final solution. The goal of a 1M solution requires exactly one mole of NaOH to be present in every one liter of the final solution volume. To determine the mass of NaOH needed, one must first calculate its molecular weight. The atomic mass of sodium is approximately 22.99 grams per mole (g/mol), oxygen is 16.00 g/mol, and hydrogen is 1.01 g/mol. By adding these values together, the molecular weight of NaOH is calculated to be about 40.00 g/mol. For one liter of a 1M solution, the required mass is calculated as: \(1 \text{ mole/liter} \times 40.00 \text{ grams/mole} \times 1.0 \text{ liter} = 40.00 \text{ grams}\).
Required Materials and Safety Precautions
Required Materials
Accurate preparation necessitates the use of a clean 1-liter volumetric flask, which is designed to contain a precise volume of solution. An analytical balance is required to weigh the 40.00 grams of NaOH pellets with high precision, typically using a weigh boat or paper. Other necessary equipment includes a glass stirring rod or magnetic stirrer, a beaker larger than the final volume, and a supply of distilled or deionized water.
Safety Precautions
Safety protocols are important because sodium hydroxide is a strong base that is highly corrosive to skin and eyes, potentially causing severe chemical burns. Mandatory personal protective equipment (PPE) includes a laboratory coat, chemical-resistant gloves, and safety goggles or a face shield. The dissolution of NaOH in water is an exothermic process, meaning it releases a significant amount of heat. This heat generation can cause the solution to splatter or the glassware to crack if the process is rushed. Consequently, it is important to use heat-resistant glassware, such as borosilicate glass, for the initial mixing.
Step-by-Step Preparation Protocol
The physical preparation begins by accurately weighing the calculated 40.00 grams of solid NaOH pellets using the analytical balance. The next step is to add a measured quantity of distilled water, typically about 500 milliliters, to the 1-liter volumetric flask or a beaker. The solid NaOH must be added slowly to the water, and it is a strict rule that water should never be added to the solid, as this can cause a violent, localized reaction and splattering.
The mixture must be continuously stirred or swirled to help the solid dissolve completely, while the temperature of the solution is carefully monitored. Because of the exothermic nature of the dissolution, the solution will become noticeably warm, and it must be allowed to cool back down to room temperature before the final volume adjustment. If the solution is diluted while still warm, the final volume will be inaccurate due to thermal expansion.
Once cooled, the solution is carefully transferred to the 1-liter volumetric flask, ensuring all traces are rinsed into the flask with small amounts of water. Finally, the flask is filled precisely to the etched line (the mark) on the neck with distilled water, using a dropper for the last few drops to ensure the bottom of the meniscus rests exactly on the mark. The flask is then stoppered and inverted several times to ensure thorough mixing and homogeneity of the 1M sodium hydroxide solution. The finished reagent must be stored in a tightly sealed container, preferably plastic or amber glass, and clearly labeled with its concentration, preparation date, and the identity of the preparer.