Name__________________
Limiting Reagents
Id#_______
Pre-Lab Discussion:
In this experiment, you will investigate the concept of limiting reagents (reactants). A limiting reagent (LR) is
the reactant that will be used up first in a chemical reaction. Once this reactant runs out, the reaction stops
and no more product can form. Thus a limiting reagent limits the amount of product that can be formed even
though another reactant(s) still remains. The other reactant(s) that remains, is said to be in excess as you have
leftover quantities of that substance that has nothing to react with to make additional product.
Identifying the limiting reagent in a problem MUST be based on the MOLAR relationships between each
reactant and one chosen product. Looking solely at the gram amounts of reactants provided is insufficient in
determining the LR.
Consider the following reax: 1 Mg(OH)2 +
35.0 g
2 HCl

1 MgCl2 + 2 HOH
43.0g
? grams
The molar relationship between the first reactant, Mg(OH)2 to the other reactant HCl is 1:2
35.0 grams of Mg(OH)2 in MOLES =0.603 moles. [ 35.0g x 1 mol]. If twice as many moles of HCl as Mg(OH)2 is
58.0 g
needed according to the stoichiometric relationship, you would require 1.21 MOLES of HCl [2 x 0.603] to react
with the Mg(OH)2, BUT you only have 1.17 moles of HCl available [ 43.0 gHCl x 1 mole = 1.17].
36.5g
Even though there are more GRAMS of HCl, the MOLAR RELATIONSHIPS show us that the HCl will run out
before the Mg(OH)2 will and therefore it is the LR.
This can be seen as well by also determining the amount of chosen product, MgCl2 that each reactant can
form:
35.0g Mg(OH)2 x 1 mol Mg(OH)2 x 1 mol MgCl2
x 95 g MgCl2=
57.3 grams of MgCl2
58 g Mg(OH)2
1 mol Mg(OH)2
1 mol MgCl2
versus
43.0g HCl
x
1 mol HCl x
36.5 g HCl
1 mol MgCl2 x 95 g MgCl2
2 mol HCl
1 mol MgCl2
55.6 grams of MgCl2
Since the HCl produces less product than the Mg(OH)2, the HCl is the limiting reagent.
In this lab, the chosen product is calcium carbonate. You will react two
soluble salts, Na2CO3 and CaCl2 in a double replacement reax, to form one
aqueous reactant that will remain in solution, and a second product, a
precipitate of calcium carbonate, the main component in chalk.
Write a balanced chemical reax for this DR reax:
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Purpose: To investigate the concept of limiting reagents and % yield using by making a common
substance, chalk, in the laboratory.
Equipment
balance
funnel
iron ring
2 flasks or beakers
wash bottle
 stirring rod
filter paper
ring stand
 pipestem triangle for funnel if needed
Materials:
Sodium carbonate Na2CO3
Calcium chloride CaCl2
Distilled water
Safety: wash your hands if skin comes in contact with any chemicals
Procedure:
1.
2.
3.
4.
5.
6.
7.
8.
mass a piece of filter paper, record in data table, and set up a filtration apparatus
using a grad. cylinder, obtain 20 ml of distilled water and pour into a 150-250 ml flask or beaker
repeat procedure 2 using a different flask or beaker and only 10 ml of distilled water
mass 2.0 grams of Na2CO3
add the 2.0 grams of Na2CO3 to the 20 ml of distilled water, swirl or stir until dissolved
mass 2.0 grams of CaCl2
add the 2.0 grams of CaCl2 to the 10 ml of distilled water, swirl or stir until dissolved
React the two aqueous solutions as follows: slowly, with stirring, add small portions of the Na2CO3
solution to the CaCl2. The mixture will begin to look frothy. Stir until the mixture vigorously until it is no
longer frothy, and a precipitate settles to the bottom (about 5 minutes).
9. Rinse the empty Na2CO3 flask or beaker and use this to “catch” the filtrate in your filtration
10. Pour the precipitate/aqueous products from your reaction flask or beaker into the funnel. Rinse the
flask with a wash bottle filled with distilled water, to make sure you leave no residue in the reaction
vessel.
11. When filtration is complete CAREFULLY move your filter paper to the perimeter of the lab area and
place on paper toweling to dry overnight. Label with your name/partner’s name.
12. Clean and rinse all equipment. Return the next day to record the mass of the filter paper with chalk.
DATA AND CALCULATIONS: SHOW ALL WORK
1. Mass of Na2CO3 used __________ Convert this to MOLES ___________
2. Mass of CaCl2 used __________
Convert this to MOLES ___________
3. Re-copy from page one, the balanced equation you wrote for this reax:
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4. Using EITHER the mole ratio of reactant to reactant OR each reactant to CaCO3 determine the limiting
reagent:
5. How much chalk SHOULD you have produced ? (What is your THEORETICAL yield ?) [If you did not
include this in your calculations from #4 above, show it here]
6. Mass of filter paper AND chalk ___________
7. Mass of filter paper alone
___________
8. Mass of chalk produced in the lab (your ACTUAL yield) = ___________
9. Percent yield FORMULA:
10. Calculation of percent yield:
11. Calculate the amount of EXCESS reagent leftover:
There is NO discussion to this lab, nor do you have to hand in the lab.
BUT you must STILL write out all calculations above because you will
be asked to SHOW me your calculations. Your assessment of this lab will be
a) Showing me you did a pre-lab in your lab note-book so that you are
familiar with the procedures BEFORE entering lab
b) Completing calculations #’s 1-5 above BEFORE the lab is performed
c) A post-lab quiz separate from or included in a quiz on limiting reagents
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