Preparing a 0.1 N Solution of Potassium Dichromate K2Cr2O7

Introduction: Potassium dichromate, (text{K}_2text{Cr}_2text{O}_7), is a strong oxidizing agent commonly used in analytical chemistry and environmental analysis. This article provides a detailed guide on how to prepare a 0.1 N solution of (text{K}_2text{Cr}_2text{O}_7) in 100 mL water, emphasizing the importance of proper calculation and handling procedures.

Step 1: Determining the Equivalent Weight

To prepare a 0.1 N normal solution of potassium dichromate, you must first determine its equivalent weight. This step involves calculating the molar mass and then using the formula for equivalent weight in a redox reaction.

Calculation of Molar Mass

The molar mass of (text{K}_2text{Cr}_2text{O}_7) is determined by adding the atomic masses of its constituent elements:

(text{K}): 39.1 g/mol, 2 atoms → 78.2 g/mol (text{Cr}): 52.0 g/mol, 2 atoms → 104.0 g/mol (text{O}): 16.0 g/mol, 7 atoms → 112.0 g/mol

Total molar mass 78.2 g/mol 104.0 g/mol 112.0 g/mol 294.2 g/mol

Equivalent Weight Calculation

During redox reactions, (text{Cr}) is reduced from 6 to 3, providing 6 equivalents of electrons. Therefore:

(text{Equivalent Weight} frac{text{Molar Mass}}{n} frac{294.2 , text{g/mol}}{6} approx 49.03 , text{g/equiv})

Step 2: Calculating the Amount Needed

To prepare 100 mL of a 0.1 N solution of (text{K}_2text{Cr}_2O_7), follow these steps:

Formula for Grams of Solute

The formula for the grams of solute required is:

(text{grams of } text{K}_2text{Cr}_2text{O}_7 text{Normality} times text{Equivalent Weight} times text{Volume (L)})

Steps to Calculate

((text{grams of } text{K}_2text{Cr}_2text{O}_7 0.1 , text{N} times 49.03 , text{g/equiv} times 0.1 , text{L} 0.4903 , text{g})

Step 3: Preparing the Solution

The steps to prepare the solution are as follows:

Weighing the Solvent

Using an analytical balance, weigh approximately (0.4903 , text{g}) of (text{K}_2text{Cr}_2O_7).

Dissolving the K2Cr2O7

Dissolve the (text{K}_2text{Cr}_2O_7) in a small volume of distilled water, less than 100 mL.

(Note: Never dissolve (text{K}_2text{Cr}_2O_7) directly in 100 mL of water, as this may cause it to form a precipitate or cause splashing.

Transferring and Adding Distilled Water

Transfer the solution to a 100 mL volumetric flask or graduated cylinder.

Add distilled water until the total volume reaches (100 , text{mL}).

Mixing Thoroughly

Mix the solution thoroughly to ensure uniformity.

Safety Measures

(Safety Note: Potassium dichromate is toxic and a known carcinogen. Always handle it with appropriate safety precautions including gloves and goggles. Work in a well-ventilated area or fume hood. Dispose of any waste according to the institution's regulations.

Conclusion

With careful calculation and adherence to proper lab protocols, you can successfully prepare a 0.1 N solution of potassium dichromate (text{K}_2text{Cr}_2text{O}_7) for various applications in analytical chemistry. Proper handling and safety measures are paramount to ensure a safe and successful experiment.

FAQs

Q: Can I prepare a 0.1 N solution in less than 100 mL water?
A: Technically, yes, but it is not recommended as it may lead to inaccurate concentration due to volume changes during the preparation process. Q: What is the main use of a 0.1 N solution of K2Cr2O7?
A: This solution is often used as a reagent in redox titrations, analytical chemistry, and environmental analysis. Q: How should I store the prepared solution?
A: Store the solution in a tightly sealed container away from direct sunlight and heat sources. Use it within a reasonable time frame and dispose of any unused solution according to the laboratory's waste management policies.