Preparing a weight by volume (w/v) solution involves dissolving a specific weight of solute in enough solvent to make a specific final volume of solution. Here's a step-by-step guide:
1. Understand the Definition of Weight by Volume (w/v)
Weight by volume (% w/v) represents the grams of solute present in 100 mL of solution. It's expressed as:
% w/v = (grams of solute / mL of solution) x 1002. Calculate the Required Mass of Solute
Determine the desired concentration (% w/v) and the final volume of the solution you want to prepare. Use the following formula to calculate the mass of solute needed:
Mass of solute (grams) = (% w/v desired / 100) x Final volume of solution (mL)Example: Let's say you want to make 200 mL of a 5% w/v NaCl (sodium chloride) solution.
Mass of NaCl = (5 / 100) x 200 mL = 10 gramsTherefore, you need 10 grams of NaCl.
3. Dissolve the Solute
- Weigh the solute: Accurately weigh the calculated mass of the solute using a balance. In our example, weigh out 10 grams of NaCl.
- Transfer to a suitable container: Transfer the weighed solute to a clean beaker or flask.
- Add solvent: Add a volume of solvent (usually water) that is less than the final desired volume. For instance, if you're aiming for 200 mL, add about 150 mL of water. This is because adding the solute will slightly increase the volume.
- Dissolve completely: Stir or swirl the mixture until the solute is completely dissolved.
4. Adjust to the Final Volume
- Transfer to a volumetric flask: Transfer the solution to a volumetric flask of the desired final volume (e.g., a 200 mL volumetric flask in our example). This provides the most accurate volume measurement. If you don't have one, use a graduated cylinder, but be aware the accuracy won't be as high.
- Add solvent to the mark: Carefully add more solvent until the bottom of the meniscus (the curved surface of the liquid) aligns with the calibration mark on the volumetric flask. For accurate volume adjustment, bring your eye level with the meniscus.
5. Mix Thoroughly
- Invert and mix: If using a volumetric flask, stopper the flask and invert it several times to ensure the solution is homogeneous. If using a graduated cylinder or beaker, mix well using a stirring rod.
Summary:
To prepare a weight by volume solution:
- Calculate the mass of solute needed based on the desired concentration and volume.
- Weigh the solute accurately.
- Dissolve the solute in a volume of solvent less than the final volume.
- Transfer to a volumetric flask (if available) and add solvent to reach the final volume.
- Mix thoroughly.
