Lab 6 – Limiting Reactant Lab
Chemistry B1A - Summer 2015
By Daniel/Cooper/Vaughan
Goal – This lab demonstrates how observations about a reaction depend on what is limiting the
reaction. You will mix two electrolytes, holding one amount constant and varying the second, to produce
a solid precipitate. Next you will develop a hypothetical balanced equation and use your observations to
determine if your data support your hypothesis. You will use your hypothetical balanced equation and
solution stoichiometry to predict the theoretical yield of the reaction, and to predict what limits the
reactions.
The reaction you will observe is a double replacement reaction, involving the reaction of two
electrolytes. Electrolytes are compounds that produce ions when dissolved in water. These ions can
move in solution so they can conduct electricity. When sodium chloride dissolves in water, the sodium
cations separate from the chloride anions.
NaCl (s) → Na+ (aq) + Cl- (aq)
When an ionic compound dissolves in water, almost all of the dissolved particles are ions. When two
dissolved compound mix, one reactant’s cation can bond to the other reactant’s anion. Sometimes a gas
AX (aq) + BY (aq) → AY (g, s) + BX (aq)
molecule is formed, sometimes water or another small molecule is formed, and sometimes a solid ionic
compound is made that precipitates or falls out of solution. In this experiment, a solid product is
formed.
Procedure
0) The sodium phosphate, Na3PO4 solution has NPFA ratings of 1 (out of 4) for health, 0 for fire, and 0
for reactivity. Cobalt (II) nitrate has 2 for health, 0 fire, and 0 for reactivity.
1) Carefully label (1-6) and weigh 6 small test tubes.
2) You will need 20 mL of 0.12 M sodium phosphate and two disposable pipettes. To each test tube use
the pump dispenser to add 3.0 mL of 0.10 M cobalt (II) nitrate. Use the other pump dispenser to
dispense 20 mL (10 mL per pump) of the colorless sodium phosphate into a clean beaker. Rinse a
buret with 2-3 mL of Na3PO4 and empty it out the bottom. Repeat this rinsing. Then add the
remaining sodium phosphate to the buret. Open the valve to get rid of any bubbles in the buret tip.
Page 2 has an example data table to record data. Use the buret to add the following amounts of
0.12 M sodium phosphate. Record the initial and final volumes to the 0.01 mL. Any recorded
volume not recorded to the hundredth of a mL will be penalized. Test tube 1 will have about
(measured to the 0.01 mL) 0.50 mL of sodium phosphate, test tube 2 will have 1.00 mL, test tube
Lab_6_CoPO4_LR S_2015
Page 1
three will have 1.50 mL, test tube 4 will have 2.00 mL, test tube 5 will have 2.50 mL, and test tube 6
will have 3.00 mL. Mix the tube contents using a vortex mixer, and centrifuge the mixture for five
minutes (or until the solid is separated from the supernatant liquid that lies above the solid
material). In a table like the following (which might look better in landscape mode), record your
observations about the solid material formed and the supernatant color after centrifuging. Save
the leftover sodium phosphate.
Test
Tube #
3.
4.
5.
6.
7.
8.
Mass of
empty
tube/g
Initial Buret
Volume/mL,
0.12 M
sodium
phosphate
Final Buret
Volume/mL,
0.12 M
sodium
phosphate
Volume sodium Observations
phosphate
added/ mL
1
On a spot plate, use a Sharpie pen to label the wells from 1-6. Use a disposable pipette to add
several drops of supernatant liquid from each test tube to a well. Rinse the pipette between samples
of supernatant liquid, and rinse the pipette several times before the next step.
You will need another milliliter of cobalt (II) nitrate. To each sample of supernatant on the spot
plate, add two drops of the original cobalt (II) nitrate solution from a dropper bottle or using a
disposable pipette. Draw a table similar to below leaving plenty of room to record your
observations in your notebook. Rinse the pipette and rinse and dry the spot plate before the next
step.
Add several drops of the supernatant to each of the 6 wells in the spot plate, and then add two
drops of the original sodium phosphate solution to each. Record your observations in your
notebook. Rinse and dry the spot plate when finished.
Test Tube
1
2
3
4
5
6
#Supernatant
=>
Supernatant
observations
after adding
Co (NO3)2
Supernatant
observations
after adding
Na3PO4
Carefully decant the remaining supernatant liquid out of each tube into an appropriate waste
container. Try not to disturb or lose any of the precipitate in the tubes.
Add 3 mL of distilled water to each tube and mix with vortex. Centrifuge each and decant. This is
called washing the precipitate.
Repeat step 7 and then put the 6 test tubes in a small beaker labeled with your lab section and you
and your partners initials, and put in an oven to dry overnight.
Lab_6_CoPO4_LR S_2015
Page 2
9. In the next lab period, remove the test tubes, cool and weigh. Record your data in your lab
notebook.
Test Tube #
1
=>
Mass of tube
with salt
product /g
Mass of tube
/g
Experimental
Mass of
precipitate
/g
2
3
4
5
6
Theoretical
Mass of
precipitatet
/g
Calculations
The following shows the calculation of the mass of cobalt (II) phosphate produced from 3.0 mL of
cobalt (II) nitrate. This calculation assumes a 2/1 ratio between cobalt phosphate and cobalt (II)
nitrate and uses the formula of CoPO4, and molar mass of 129.56 for cobalt (II) phosphate. All of
these assumptions are incorrect. The student needs to use the product formula, correct molar ratio,
and molar mass. The first step has converted 0.10 M, (or 0.10 moles/Liter) into 0.10 moles/1000
mL.
3.0 𝑚𝐿 𝐶𝑜(𝐼𝐼)𝑛𝑖𝑡𝑟𝑎𝑡𝑒 𝑋
0.10 𝑚𝑜𝑙 𝐶𝑜(𝐼𝐼)𝑛𝑖𝑡𝑟𝑎𝑡𝑒
2 𝐶𝑜 𝑃𝑂4
129.56 𝑔 𝐶𝑜𝑃𝑂4
𝑋 1 𝐶𝑜 (𝑁𝑂3)2 𝑋 1 𝑚𝑜𝑙 𝐶𝑜𝑃𝑂4
1000 𝑚𝐿
= g Co PO4
You will do similar calculations for each volume of sodium phosphate. By comparing the theoretical
yield of product from cobalt (II) nitrate versus sodium phosphate, you can predict how much
product was made.
Report
This report will follow the outline given for a formal report. Any tables should be typed in your
report. Please include the following in the order given in your discussion of results.
1) Balance the electrical charges in the products, then balance the equation. Neatly write the
equations as
a) The molecular equation, labeling the physical states (s, L, g, aq) of all reactants and products.
b) The ionic equation, labeling the physical states, and electric charge of all ions.
c) The net ionic equation, labeling the physical states, and electric charge of all ions.
d) Briefly justify your choice of precipitate.
Lab_6_CoPO4_LR S_2015
Page 3
2) Use your balanced equation to calculate the volume of 0.12 M Na3PO4 to stoichiometrically react
with the 3.0 mL of Co (NO3)2. Show your work.
3) Discuss how your supernatant color in the 6 different test tubes supports your calculated result in
#2. For example, if you used less than the volume of sodium phosphate calculated in #2, what would
be the limiting reactant and does the observed supernatant color support this.
4) Calculate the theoretical yield of solid precipitate if the 3.0 mL of Co (NO3)2 limits the reaction. Show
your work.
5) Calculate the theoretical yields of solid precipitate if the different volumes of 0.12 M Na3PO4 limits
the reactions. You only need to show your work for the 0.5 mL reaction and show your volume of
sodium phosphate and the calculated results for the other 5 calculations. Use your experimentally
measured volumes of sodium phosphate.
6) Include a graph of students’ experimental precipitate mass versus volume of sodium phosphate, and
discuss how the mass of precipitate should change as volume of sodium phosphate increases.
Comment on if the data supports your calculated result in #2. Discuss how the formation of product
hydrates would affect the data.
7) Your conclusions should tie together and summarize the supernatant color, supernatant test,
product mass and the balanced equation.
Lab_6_CoPO4_LR S_2015
Page 4