Lab Activity 17: Mole to Mass to Mole Stoichiometry

PbNO3(aq) + K2Cr2O7(aq) --> PbCr2O7(s) + 2KNO3(aq)

Pre-Lab Discussion:
In a balanced chemical reaction you can ratio 2 different substances in the unit of the mole. In this lab you will weigh the mass of the two reactants, both will be considered your knowns. You will then react them and collect your resulting products, you will also determine if the correct number of moles of product was collected as defined in the above reaction. There are two quantities you will consider:

1)The quantity of product you measure in the lab is called the Actual-or-Experimental Yield because you actually weigh it in the lab. This will be recorded in your lab notebook and defines a reality.

2)The Theoretical Yield is what you CALCULATE the quantity of product should be in accordance to the reaction stoichiometry. This is theoretical as it is a calculation you execute on paper or in a calculator.

It is important to note that you cannot measure a mole directly. In this lab you will weigh your substances in grams, you must then convert to moles for your calculations. Remember you cannot weigh moles in the lab nor should you ratio grams in reactions!

Read this entire lab handout and type a pre-lab with an empty Observations section to fill-in during lab. Also, before you come to lab, read the procedures, and calculate the theoretical mass of lead dichromate expected. Do this twice, once starting with 1.33g lead nitrate, the second time starting with 1.00g potassium dichromate, and compare your results.

Purpose:
To react a known amount of moles of PbNO3(aq) with a known amount of moles of K2Cr2O7(aq). The Percentage Yield of the product PbCr2O7(s) will then be calculated by using mole ratio stoichiometry theory.

Safety:
All of these substances are toxic you must wear gloves and an apron. The potassium dichromate is caustic therefore you must wear goggles. Be certain to wash you hands before leaving the lab.

Procedures:
(NOTE – record all masses to three significant figures)

1. Mass 1.00 grams of potassium dichromate solid, dissolve it into 30. mL of warm water in a clean 100. mL beaker.
2. Mass 1.13 grams of lead nitrate, dissolve it into 30. mL of warm water in a 100. mL beaker.
3. Slowly mix the two solutions into a third clean 100. mL beaker, allow the resulting lead dichromate precipitate to settle for about 3 min.
4. While the solid product settles, prepare a filtration apparatus. Be certain you have labeled your filter paper in pencil and then mass it to three significant figures.
5. Decant about half of the liquid that covers your solid product. This is called the supernatant liquid, if you were not asked to save this to a clean beaker for later study, you may discard this liquid.
6. Suspend the solid product with the remaining supernatant liquid by swirling the beaker. Slowly pour the suspension into the filter paper cone using a stirring rod to direct the flow. Wash any remaining solid out of the beaker with 5 mL of COLD water, this also flushes the potassium nitrate out of the lead dichromate. Do not use too much wash water as this will take to long to filter. IMP- you will not recover your entire product from the beaker, ask your teacher if you have collected enough of it!
7. Using lab tweezers, carefully remove your filterpaper cone containing the solid product and place it in the drying oven. When it is returned to you, mass the paper and product.

-Michael Caifa 10/05 St Francis Preparatory

Data:

• Create a data table(s) to organize all your measurements and calculated results.

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