WKS – Chem Honors
Limiting & Excess Reactants
NAME Answer Key
Period
Date
1) What is meant by the limiting reactant? Excess reactant? Why is it necessary to identify the limiting
reactant when you want to know how much product will form in a chemical reaction?
The limiting reactant is completely consumed during a chemical reaction. The excess reactant has some
remaining. The limiting reactant determines the amount of product that can theoretically be produced
2) The unbalanced equation representing the production of tetraphosphorus trisulfide (P4S3), a substance
used in some match heads, is
8 P4(s) + 3 S8(s) → 8 P4S3(s).
a. Balance this synthesis reaction using the lowest whole-number coefficients.
b. When 6.00 mol P4 and 6.00 mol S8 react, which is the limiting reactant?
Calculate and compare equivalents:
6.00 mol P4
6.00 mol S8
= 0.75 equiv. P4 ;
= 2.00 equiv. S8 ; 0.75 < 2.00, so P4 is limiting.
8 mol P4
3 mol S8
c. Which reactant is in excess, and how many moles remain after the reaction is complete?
d. How many grams of P4S3 are produced?
3) Trisilicon tetranitride, a ceramic that has orthopedic applications, is formed by reacting silicon with
nitrogen gas at high temperature:
3 Si (s) + 2 N2 (g) →
Si3N4 (s)
a. Balance this synthesis reaction using the lowest whole-number coefficients.
b. What is the limiting reactant if 3.17 g of Si and 2.55 g of N2 react?
1 mol Si
1 mol N 2
3.17 g Si ×
= 0.113 mol Si; 2.55 g N 2 ×
= 0.0910 mol N 2
28.09 g Si
28.02 mol N 2
0.113 mol Si
0.0910 mol N 2
= 0.0377 equiv. Si;
= 0.0455 equiv. N 2 ; 0.0377 < 0.0455 so Si is limiting.
3 mol Si
2 mol N 2
c. What is the excess reactant, and how many grams remain after the reaction is completed?
d. How many grams of Si3N4 can be produced from these reactants?
4) In blast furnaces, iron(III) oxide (Fe2O3) reacts with (is reduced by) gaseous carbon monoxide (CO) to
form liquid iron and gaseous carbon dioxide:
Fe2O3 (s) + 3 CO (g) → 2 Fe (l)+ 3 CO2 (g)
a. Balance the equation with the lowest whole-number coefficients.
b. If 100. g of iron(III) oxide and 56.0 g of carbon monoxide are reacted, which reactant is limiting?
1 mol Fe 2O3
1 mol CO
100. g Fe 2O3 ×
= 0.626 mol Fe 2O3 ; 56.0 g CO ×
= 2.00 mol CO
159.7 g Fe 2O3
28.01 mol CO
0.626 mol Fe 2O3
2.00 mol CO
= 0.626 equiv. Fe 2O3 ;
= 0.667 equiv. CO; 0.626 < 0.667, so Fe 2O3 is limiting
1 mol Fe 2O3
3 mol CO
c. How much excess reactant (in g) remain after the reaction is completed?
d. How many grams of iron are produced in this reaction?
5) One industrial process for producing nitric acid begins by combusting ammonia gas (NH3) in oxygen
gas to form nitrogen monoxide gas (NO) and liquid water.
4 NH3 (g) + 5 O2 (g) → 4 NO (g) + 6 H2O (l)
a. Balance the equation with the lowest whole-number coefficients.
b. Which reactant is limiting if 4.20×104 g of NH3 and 1.31×105 g of O2 are available?
1 mol NH 3
1 mol O 2
4.20 × 104 g NH 3 ×
= 2460 mol NH 3 ; 1.31× 105 g O 2 ×
= 4090 mol O 2
17.04 g NH 3
32.00 mol O 2
2460 mol NH 3
4090 mol O 2
= 615 equiv. NH 3 ;
= 818 equiv. O 2 ; 615 < 818, so NH 3 is limiting
4 mol NH 3
5 mol O 2
c. Which reactant is in excess, and how many grams remain after the reaction is completed?
d. How many grams of NO will be produced in this reaction?
Limiting & Excess Reactants WS
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