Formulas and Moles
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A Chemical Reaction of Cobalt(II) Nitrate with Sodium Phosphate
- A Lesson in Stoichiometry From a balanced chemical equation we can deduce or calculate many things. For example, a balanced
chemical equation can be used to find the ratio of how much of each substance the reaction is going to use or
produce, in moles. Stoichiometry is the name for the procedure that is used when these amounts are compared to
each other. When calculating the stoichiometric ratio from a balanced chemical equation, we must always work in
mole amounts. From the reaction below, for instance, we see that we will always use twice as much KI as Pb(NO3)2,
in moles not grams, so the stoichiometric factor is 2:1for KI to Pb(NO3)2.
Pb(NO3)2 (aq) + 2KI (aq) ! PbI2 (s) + 2KNO3 (aq)
If we begin with the mass of the substances in grams we will always need to convert that mass to moles
and work from there. Consequently, we must know, or be able to derive, the molar mass of the compounds we are
working with. The molar mass of KI is 166 g/mol, for Pb(NO3)2 the molar mass is 331.2 g/mol, PbI2 is 461 g/mol,
and KNO3 is 101.1 g/mol.
If we are given 5.0g of Pb(NO3)2, the amount we have in moles is easily calculated through dimensional
analysis, and is 0.015mol. If we have excess KI in the reaction, we could determine, using stoichiometry, the
amount of PbI2 we should be able to synthesize. The stoichiometric ratio of Pb(NO3)2 to PbI2 in the reaction is 1:1.
So we should simply see 0.015 moles of PbI2 produced, which is 6.9 grams – based on PbI2 having a molar mass of
461 g/mol. As you can readily see, the mass of PbI2 produced is not the same as the mass of the Pb(NO3)2 used,
even though the stoichiometric factor was 1:1. This is why all the mass figures must first be converted from grams
to moles.
The prediction that we should be able to produce 6.9g of PbI2 is called the theoretical yield for the reaction.
It is the maximum amount we should be able to see produced if the reaction proceeds as written in the chemical
equation. The amount we actually get is divided by the theoretical yield and multiplied by 100 to obtain what is
called the percent yield.
In this experiment, we will react cobalt(II) nitrate with sodium phosphate in an aqueous solution to produce
solid cobalt(II) phosphate and aqueous sodium nitrate. The unbalanced chemical equation looks like this:
Co(NO3)2 + Na3PO4 ! Co3(PO4)2 + NaNO3
Goals: The students will
1. explore solution chemistry and the production of a precipitate
2. use observations to predict the experimental stoichiometric relationship of reactants to products
3. calculate the percent yield for this reaction
4. utilize dimensional analysis to calculate mass percent and conversion of quantities
Prelab Questions
1. When figuring stoichiometric relationships, what must always be done first?
2.
Re-write the chemical equation above in its complete and balanced form, indicating whether the compound is
aqueous or solid.
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Stoichiometry
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3.
Define stoichiometry.
4.
Calculate the molar mass of each of the compounds in the reaction.
______________
Co3(PO4)2
Co(NO3)2
Co(NO3)2 • 6H20
NaNO3
______________
______________
Na3PO4
_______________
_______________
Safety:
•
Goggles and lab aprons must be worn at all times
•
These chemicals can cause great physical harm, wash them off immediately if you get any on you!
•
Dispose of your chemicals ONLY in the designated containers, do not put anything down the drain or in the
garbage.
•
Wash your hands at the completion of the experiment.
Procedure:
1. Carefully weigh approximately 2.0g of the cobalt(II) nitrate hexa-hydrate [Co(NO3)2 • 6H2O)]crystal on a
weighing paper on the balance.
2.
Record the exact mass of the cobalt(II) nitrate hexa-hydrate in the data table and transfer it to a 100 mL
beaker. Also record the appearance of the crystals in the Observations section below the data table.
3.
Measure approximately 20.0 mL of distilled water into the beaker containing the cobalt(II) nitrate hexahydrate.
4.
Stir the contents of the beaker with a stirring rod until the cobalt(II) nitrate hexa-hydrate has completely
dissolved. Record your observations.
5.
On another weighing paper, measure approximately 1.0 g of sodium phosphate [Na3PO4]. Record the exact
mass in the data table and transfer to a 50 mL beaker. Record the appearance of the crystals in the
Observations section.
6.
Again, measure approximately 20.0 mL of distilled water into the beaker containing the sodium phosphate.
Stir until the sodium phosphate has completely dissolved. Record your observations.
7.
Slowly and carefully pour the sodium phosphate solution into the cobalt(II) nitrate hexa-hydrate solution.
DO NOT STIR, SWIRL, OR MOVE THE PRECIPITATE AROUND. Record your observations.
8.
Using a glass funnel, stopper collar, filter paper, and filtering flask, set up the filtering apparatus as
described below.
a. Weigh the filter paper and record the mass in the data table.
b. Put the glass funnel and stopper collar into the mouth of the filtering flask.
c. Gently fold the filter paper in half. Gently fold it in half again (it should now be the shape of a quarter of
a circle).
d. Open one side of the filter paper so that it looks like a cone. Put the filter paper into the funnel.
e. Lightly wet the filter paper with distilled water while continuously squeezing the suction bulb – this
holds the paper in place. (You’ll see the water going out of the funnel into the filtering flask.)
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Stoichiometry
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9.
CAREFULLY pour the mixture from your 100 mL beaker into the glass funnel. DO NOT ALLOW THE
LIQUID TO COME ANY HIGHER THAN 0.5CM FROM THE TOP OF THE FILTER PAPER.
10. Using as little water as possible (this is very important – adding too much water will make your overall
filtering time much longer), flush any remaining solid with distilled water that may still be in the 100mL
beaker into the funnel.
11. Suction all the fluid away, by continuously squeezing the suction bulb, until the solid appears semi-dry.
Then, ask your teacher to gently squirt a small amount of acetone all over the solid. Suction off until the
solid is semi-dry again.
12. Carefully remove the filter paper (with the solid precipitate on it) and place it on a paper towel.
13. With care, take your precipitate on the paper towel, and place it in a warm dry place.
14. When your precipitate is completely dry, probably the next day (your teacher will help you determine that),
take your filter paper and product and weigh it, recording the mass in the data table.
Data and Observations:
Mass of cobalt(II) nitrate hexa-hydrate …………..
Moles of cobalt(II) nitrate hexa-hydrate...
Mass of sodium phosphate ……………………..
Moles of sodium phosphate …………..
Mass of dry filter paper……………………………
Mass of filter paper & solid (after drying)……......
Mass of solid Co3(PO4)2 produced………………..
Moles of Co3(PO4)2 ………………………………….
__________
__________moles Co(NO3)2 •6H20
__________
__________moles Na3PO4
__________
__________
__________
__________moles Co3(PO4)2
Observations:
1. Describe the appearance of the dry cobalt(II) nitrate hexa-hydrate crystals and compare them to the appearance
of the dry sodium phosphate crystals.
2.
Describe the appearance of the cobalt(II) nitrate hexa-hydrate solution.
3.
Describe the appearance of the sodium phosphate solution.
4.
Describe the appearance of what occurred when the solutions were mixed.
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Post Lab
Show all formulas, set ups and answers with units.
1. Calculate, using dimensional analysis, the number of moles of Co(NO3)2 • 6H20 and Na3PO4 you used in
the reaction. (NOTE: You should have two answers.)
2.
Why does the balanced chemical equation leave out the •6H2O part of the Co(NO3)2 • 6H20?
3.
What is the stoichiometric ratio of Na3PO4 to Co(NO3)2 • 6H20, as given by the balanced equation? Using
the amounts from question #1, calculate which reagent is in limited supply?
4.
Identify the limiting reagent in the reaction. Explain why it is the limiting reagent.
5.
Using the experimental data (the data you just collected), calculate the number of moles of Co3(PO4)2
produced (the actual).
6.
The theoretical stoichiometric ratio can be determined by the balanced chemical equation. (Look back at it
if you need to.) Calculate the theoretical mole ratio of cobalt(II) nitrate hexa-hydrate to cobalt(II)
phosphate. (NOTE; we are comparing something on the left side of the equation to something on the right
side of the equation – a reactant with a product.)
THEORETICAL MOLE RATIO: ______moles Co(NO3)2 • 6H20 to ______moles Co3(PO4)2
Hint: figure out how many moles of Co(NO3)2 • 6H20 it takes to make 1mole of Co3(PO4)2
7.
Using the calculated amount of the limiting reagent, in moles, calculate the amount of cobalt(II) phosphate
that should be theoretically produced by the reaction, also in moles. Solve by dimensional analysis.
8.
Calculate the percent yield of this reaction for the production of cobalt(II) phosphate.
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Stoichiometry
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9.
Calculate the percent error of this experiment for the production of cobalt(II) phosphate.
10. From the mole amounts in your data table, you can now calculate the experimental mole ratio for the
reaction (which is the experimental stoichiometric relationship). This can be accomplished by comparing
the mole value of Co3(PO4)2 and the mole value of the limiting reagent.
Moles of Co3(PO4)2 : ___________
Moles of the limiting Reagent [Co(NO3)2 • 6H20] : ___________
Divide both quantities by whichever one is smaller. The ratio is then given as;
EXPERIMENTAL RATIO -
_______________ to
_______________
How does this compare to your theoretical mole ratio?
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