How will you determine the %age purity of KD Cr:O when you are provided with the 0.1 M Mohr's salt solution is provided

 To determine the percentage purity of potassium dichromate (K₂Cr₂O₇) using a 0.1 M solution of Mohr's salt (ferrous ammonium sulfate, Fe(NH₄)₂(SO₄)₂·6H₂O), you can perform a redox titration. Here is a step-by-step outline of the procedure:

Materials Needed

• Potassium dichromate sample
• 0.1 M Mohr's salt solution
• Dilute sulfuric acid (H₂SO₄)
• Burette
• Pipette
• Conical flask
• Indicator (typically diphenylamine sulfonate or N-phenylanthranilic acid)

Procedure

1. Prepare the Sample Solution: Accurately weigh a sample of the potassium dichromate and dissolve it in distilled water to prepare a solution of known concentration.

2. Set Up the Titration:

   • Fill the burette with the 0.1 M Mohr's salt solution.
   • Pipette a known volume of the potassium dichromate solution into a conical flask.
   • Add an appropriate amount of dilute sulfuric acid to the flask to maintain an acidic medium (usually 1-2 mL of 1 M H₂SO₄).

3. Add the Indicator:

   • Add a few drops of the chosen redox indicator to the flask. Diphenylamine sulfonate is commonly used, which changes color at the endpoint of the titration.

4. Titrate:

   • Titrate the potassium dichromate solution with the 0.1 M Mohr's salt solution, swirling the flask continuously, until you reach the endpoint indicated by a distinct color change.

5. Calculate the Amount of Mohr's Salt Used:

   • Record the volume of Mohr's salt solution used to reach the endpoint.

Calculation

1. Determine the Moles of Mohr's Salt Used:

   $$\text{Moles of Mohr's salt} = \text{Volume (L) of Mohr's salt} \times 0.1 \, \text{M}$$

2. Calculate the Moles of Potassium Dichromate: The balanced redox reaction between potassium dichromate and Mohr's salt in acidic medium is:

   $$\text{Cr}_2\text{O}_7^{2-} + 14\text{H}^+ + 6\text{Fe}^{2+} \rightarrow 2\text{Cr}^{3+} + 7\text{H}_2\text{O} + 6\text{Fe}^{3+}$$

   According to the stoichiometry of the reaction, 1 mole of $\text{Cr}_2\text{O}_7^{2-}$ reacts with 6 moles of $\text{Fe}^{2+}$. Therefore:

   $$\text{Moles of } \text{Cr}_2\text{O}_7^{2-} = \frac{\text{Moles of Mohr's salt}}{6}$$

3. Determine the Mass of Potassium Dichromate: Using the molar mass of potassium dichromate (294.18 g/mol):

   $$\text{Mass of } \text{Cr}_2\text{O}_7^{2-} = \text{Moles of } \text{Cr}_2\text{O}_7^{2-} \times 294.18 \, \text{g/mol}$$

4. Calculate the Percentage Purity: Compare the mass of potassium dichromate obtained from the titration to the initial mass of the sample:

   $$\text{Percentage purity} = \left( \frac{\text{Mass of pure } \text{Cr}_2\text{O}_7^{2-}}{\text{Mass of sample}} \right) \times 100$$

Example Calculation

Let's say you weighed a 0.5 g sample of potassium dichromate and used 20 mL of 0.1 M Mohr's salt solution in the titration.

1. Moles of Mohr's salt:

   $$\text{Moles of Mohr's salt} = 0.02 \, \text{L} \times 0.1 \, \text{M} = 0.002 \, \text{moles}$$

2. Moles of potassium dichromate:

   $$\text{Moles of } \text{Cr}_2\text{O}_7^{2-} = \frac{0.002}{6} = 0.000333 \, \text{moles}$$

3. Mass of potassium dichromate:

   $$\text{Mass of } \text{Cr}_2\text{O}_7^{2-} = 0.000333 \times 294.18 \, \text{g/mol} = 0.098 \, \text{g}$$

4. Percentage purity:

   $$\text{Percentage purity} = \left( \frac{0.098}{0.5} \right) \times 100 = 19.6\%$$

Thus, the potassium dichromate sample is 19.6% pure.