Question

When preparing a solution of known concentration, explain why one must first dissolve the solid completely before adding enough solvent to fill the volumetric flask to the mark.

Short answer

Complete dissolution ensures uniform solute distribution; filling to the mark achieves precise concentration.

Step-by-step solution

Understanding the Problem

To prepare a solution of a known concentration, it means you need to dissolve a specific amount of solid chemical (solute) in a solvent to reach a precise concentration.

Importance of Full Dissolution

First, ensure the solid solute is completely dissolved in a portion of the solvent. This ensures that the solute is evenly distributed throughout the solution, which is critical for achieving the correct concentration.

Solvent Addition and Final Adjustment

After the solute has completely dissolved, add more solvent, gradually filling the volumetric flask to the calibration mark. This mark indicates the desired total volume of the solution.

Ensuring Accurate Concentration

The complete dissolution of the solute before reaching the final volume ensures that no solid particles remain undissolved, which would otherwise lower the actual concentration of the solution. Adjusting to the marked volume afterward ensures the concentration is accurate as calculated.

Key concepts

Solute Dissolution

When preparing a solution, the first critical step is the dissolution of the solute in a solvent. This means the solid substance, or solute, must be fully dissolved into the liquid to ensure an even distribution.

• Start by adding a small, measureable portion of solvent to the solute.
• Stir the mixture thoroughly to make sure all solute particles dissolve.

Once the solute is fully dissolved, you avoid uneven concentration throughout the solution. Any remaining particles indicate incomplete dissolution and can affect the accuracy of your calculations.

Volumetric Flask Use

A volumetric flask is an essential piece of laboratory glassware. It's specifically designed to hold a precise volume of liquid.

• The flask is marked with a line on its neck to indicate the total volume it can hold.
• Its shape helps ensure that solutions are well mixed.

Using a volumetric flask ensures that when you fill to the line, you can trust that you have the exact volume needed for your concentration calculations.

Concentration Accuracy

Achieving accurate concentration is all about precision in mixing and measuring.

• Begin by fully dissolving the solute before making up the solution to final volume. This avoids errors in concentration.
• Make sure to adjust the volume level to the calibration mark on the volumetric flask for accuracy.

Any deviation from these steps could result in incorrect concentrations, which would affect experimental outcomes, possibly leading to errors if concentrations are used in further reactions or computations.

Solvent Addition

After ensuring that all the solute is dissolved, the next step is careful addition of more solvent until the solution reaches the mark on the volumetric flask.

• This process is critical for maintaining the desired concentration.
• Always add the solvent gradually. Rapid addition can make it easy to overshoot the mark, leading to dilution issues.

Filling the flask carefully to the mark ensures that the final volume contains the correct amount of solute, based on the specific concentration needed for the experiment or process.