How to do Gravimetric Analysis in Chemistry (with calculations and examples!)

Summary

In this video, Jason Neal, a chemistry teacher, introduces gravimetric analysis, a lab technique for determining the concentration of a solution. Through the experiment, he demonstrates how to isolate iodide ions using lead nitrate, form a precipitate, and calculate the molarity of potassium iodide. Neal emphasizes the importance of precision, proper rinsing, and understanding the role of ions in the process. By the end, viewers will gain a clear understanding of gravimetric analysis and how to perform calculations to find solution concentration using this method.

Takeaways

• 😀 Gravimetric analysis is a technique used to determine the concentration of a solution through the measurement of a precipitate's mass.
• 😀 The concentration of a solution remains the same regardless of the container's size, provided the volume remains constant.
• 😀 Molarity is defined as moles of solute per liter of solution, and the goal of the experiment is to find the molarity of the potassium iodide solution.
• 😀 The volume of the potassium iodide solution is measured using a graduated cylinder (9.45 mL), which is then converted to liters (0.00945 L).
• 😀 Distilled water is added to ensure all potassium iodide is transferred, but it doesn't affect the number of moles in the solution.
• 😀 Lead nitrate is added to the potassium iodide solution, forming a yellow precipitate of lead iodide (PbI₂), which is insoluble in water.
• 😀 The precipitate is separated from the solution using filtration, with careful rinsing to remove excess ions (potassium, nitrate, extra lead).
• 😀 The filtered lead iodide is dried, and its mass is measured (2.44 grams), from which the mass of the lead iodide alone (1.70 grams) is calculated.
• 😀 The moles of lead iodide are calculated using its molar mass (461 g/mol), resulting in 0.00369 moles of PbI₂.
• 😀 The moles of iodide are twice the moles of lead iodide, so the moles of iodide are calculated to be 0.00738 moles.
• 😀 The molarity of the original potassium iodide solution is calculated by dividing the moles of iodide by the volume in liters (0.00945 L), yielding a molarity of 0.781 M.

Q & A

What is gravimetric analysis?

Gravimetric analysis is a lab technique used to determine the amount of a substance by measuring the mass of a precipitate that forms during a chemical reaction.

Why is it important to wear chemical splash goggles in the lab?

Chemical splash goggles are crucial for protecting your eyes from potential splashes of chemicals that could cause injury or harm.

What is the significance of using the same concentration of a solution, regardless of the container size?

The concentration of a solution remains the same regardless of container size because it is based on the amount of solute per unit volume, not the total volume.

How is molarity calculated in this experiment?

Molarity is calculated by dividing the moles of solute by the volume of the solution in liters. In the example, the molarity is found by calculating the moles of potassium iodide and dividing by the volume of the solution (9.45 mL converted to liters).

Why is it necessary to rinse the graduated cylinder with distilled water?

Rinsing the graduated cylinder ensures that all of the potassium iodide solution is transferred into the new container, avoiding any loss of the substance and ensuring accurate measurements.

Why is lead nitrate used in this experiment?

Lead nitrate is used to react with potassium iodide to form lead iodide, a yellow precipitate, which helps in determining the amount of iodide in the solution.

What is the significance of the yellow precipitate formed in the reaction?

The yellow precipitate of lead iodide is important because it represents the iodide ions from the potassium iodide solution that have reacted with lead ions, allowing for the calculation of iodide and subsequently the potassium iodide concentration.

Why must the precipitate be rinsed thoroughly during the filtration process?

The precipitate must be rinsed thoroughly to remove any remaining dissolved ions (like potassium or nitrate ions) that could introduce errors in the final measurement of the lead iodide.

What is the purpose of drying the filter paper and precipitate?

Drying the filter paper and precipitate ensures that all moisture is removed, allowing for an accurate mass measurement of the lead iodide, which is necessary for calculating the molarity of the original solution.

How is the amount of potassium iodide calculated from the mass of lead iodide?

The mass of lead iodide is used to calculate the moles of lead iodide, and because the ratio of iodide to lead iodide is 2:1, the moles of iodide are twice that of lead iodide. This value is used to determine the moles of potassium iodide, which then leads to the molarity calculation.