# Limiting and Excess Reagent Calculations - plaza.stfx.com

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```					           Calculating Mass of Excess Reagents
Recall:
A limiting reagent is the reactant whose entities are completely
consumed in a reaction. It is the one that runs out first, and as a result,
stops the reaction.
An excess reagent is any reactant that is left over at the conclusion of
the reaction.

HOW MUCH REAGENT IS ENOUGH TO BE EXCESS ??

Rule of thumb: Do stoichiometry to calculate the minimum mass
You decide to test the method of stoichiometry using the reaction of
2.00 g of copper (II) sulfate in solution with an excess of sodium
hydroxide in solution. What would be a reasonable mass of sodium
hydroxide to use?

1.10 g NaOH(aq)
Identifying Limiting and Excess Reagents
Identify the limiting reagent by:
1) converting each reagent to chemical amount (i.e. number of moles)
2) using the mole ratio to compare the required amount of the other
reagent with the amount actually present
The excess reagent is the one that is not the limiting reagent (duh).

If 10.0 g of copper is placed in a solution of 20.0 g of silver nitrate,
which reagent will be the limiting reagent?

AgNO3 (aq)
In the reaction of a 10.0 g sample of copper with 20.0 g of silver nitrate
in solution, what mass of copper will be in excess? What mass of silver
will be produced?

nAgNO3 (aq) = 0.118 mol       (from previous example)

6.26 g excess copper

12.7 g silver
In an experiment, 26.8 g of iron (III) chloride in solution is combined
with 21.5 g of sodium hydroxide in solution. Which reactant is in
excess, and by how much? What mass of each product will be
obtained?

17.7 g Fe(OH)3 (s)
1.7 g excess NaOH(aq)

29.0 g NaCl(aq)
Another Example
If 10.1 g of magnesium and 2.87 g of hydrogen chloride gas are reacted,
what is the limiting reagent? How much excess reagent remains? How
many grams of gas will be produced?

Mg (s) + 2 HCl(g)  MgCl2(s) + H 2(g)
10.1 g         2.87 g                                m
24.31 g/mol    36.46 g/mol                      2.02 g/mol

limiting reagent is HCl(g)          9.04 g excess Mg(s)

0.0795 g H2 (g)
Yet Another Example
If 10.3 g of aluminum are reacted with 51.7 g of aqueous copper (II)
sulfate, how much copper will be produced? How much excess reagent
will remain?

2 Al(s) + 3 CuSO 4(aq)  3 Cu (s) + Al2 SO 4 3(aq)
10.3 g          51.7 g             m
26.98 g/mol    159.62 g/mol      63.55 g/mol

20.6 g Cu(s)

4.48 g excess Al(s)
 Read pgs. 320 – 324

 pgs. 321, 324 Practice #’s 1 – 5

 pg. 327 Section 8.3 Questions #’s 1 – 9

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