Excess Reactant Examples in Chemistry

Start with the recap, study the fully worked examples, then use the practice problems to check your understanding of Excess Reactant.

This page combines explanation, solved examples, and follow-up practice so you can move from recognition to confident problem-solving in Chemistry.

Concept Recap

The reactant that remains after a reaction stops because the limiting reactant has been used up.

If one ingredient runs out first, the other one is left over.

Read the full concept explanation →

How to Use These Examples

Read the first worked example with the solution open so the structure is clear.
Try the practice problems before revealing each solution.
Use the related concepts and background knowledge badges if you feel stuck.

What to Focus On

Core idea: Excess Reactant starts with the given amount, names the substance, and chooses the conversion factor that cancels the old unit.

Common stuck point: Students often know a formula related to excess reactant but skip the recognition step: Am I using a mole bridge, molar mass, formula ratio, or balanced-equation ratio to connect measured amounts? That leads to a correct-looking substitution attached to the wrong chemical model.

Sense of Study hint: Ask: Am I using a mole bridge, molar mass, formula ratio, or balanced-equation ratio to connect measured amounts?

Worked Examples

Example 1

easy

Mg + 2HCl \rightarrow MgCl_2 + H_2

2\text{ mol}

Mg

and

5\text{ mol}

HCl

react. Identify the excess reactant.

Answer

Mg

First step

2\text{ mol}

Mg

requires

2 \times 2 = 4\text{ mol}

HCl

See the full worked solution + why-it-works coaching

SetupKey insightWhy it worksCommon pitfallConnection

Unlock answer keys One Family plan — every worked solution, all subjects

Example 2

medium

2Ag + Cl_2 \rightarrow 2AgCl

0.4\text{ mol}

Ag

and

0.3\text{ mol}

Cl_2

react. Find the moles of excess reactant remaining.

Example 3

medium

2NaOH + H_2SO_4 \rightarrow Na_2SO_4 + 2H_2O

0.6\text{ mol}

NaOH

and

0.4\text{ mol}

H_2SO_4

react. Find the excess reactant remaining.

Example 4

medium

2KMnO_4 + 16HCl \rightarrow 2KCl + 2MnCl_2 + 5Cl_2 + 8H_2O

0.2\text{ mol}

KMnO_4

and

2\text{ mol}

HCl

react. How many moles of

HCl

remain?

Example 5

hard

2AgNO_3 + CaCl_2 \rightarrow 2AgCl + Ca(NO_3)_2

0.6\text{ mol}

AgNO_3

and

0.4\text{ mol}

CaCl_2

react. Find excess reactant remaining.

Example 6

hard

4Fe + 3O_2 \rightarrow 2Fe_2O_3

11.2\text{ g}

Fe

(

M=56

) and

4.8\text{ g}

O_2

(

M=32

) react. How many grams of excess reactant remain?

Example 7

challenge

2KClO_3 \rightarrow 2KCl + 3O_2

... since decomposition has only one reactant, instead consider

3Mg + N_2 \rightarrow Mg_3N_2

. If

0.9\text{ mol}

Mg

and

0.4\text{ mol}

N_2

react, find excess remaining and moles of

Mg_3N_2

formed.

Practice Problems

Try these problems on your own first, then open the solution to compare your method.

Example 1

medium

2H_2 + O_2 \rightarrow 2H_2O

5\text{ mol}

H_2

and

4\text{ mol}

O_2

react. How many moles of excess reactant remain?

Example 2

easy

In a reaction,

A

is the limiting reactant and

B

is left over after the reaction stops. Which is the excess reactant?

Example 3

easy

5\text{ mol}

of a reactant is provided but only

3\text{ mol}

is consumed, how much is in excess?

Example 4

easy

2H_2 + O_2 \rightarrow 2H_2O

, you have

2\text{ mol}

H_2

and

2\text{ mol}

O_2

. Which is in excess?

Example 5

easy

After a reaction,

0.8\text{ mol}

of a reactant remains unreacted. What is this leftover called?

Example 6

easy

N_2 + 3H_2 \rightarrow 2NH_3

1\text{ mol}

N_2

reacts with

3\text{ mol}

H_2

. With

1\text{ mol}

N_2

and

5\text{ mol}

H_2

, which is excess?

Example 7

easy

You supply

10\text{ g}

of a reactant and

4\text{ g}

reacts. What mass is in excess?

Example 8

easy

Does the excess reactant or the limiting reactant determine how much product forms?

Example 9

easy

In a

1:1

reaction

A + B \rightarrow C

, you have

4\text{ mol}

A

and

6\text{ mol}

B

. Which is excess and by how much?

Example 10

medium

2Na + Cl_2 \rightarrow 2NaCl

, you mix

4\text{ mol}

Na

with

3\text{ mol}

Cl_2

. Which is excess and how much remains?

Example 11

medium

C + O_2 \rightarrow CO_2

24\text{ g}

carbon reacts with

64\text{ g}

oxygen. Which is excess? (

C=12

O_2=32\text{ g/mol}

)

Example 12

medium

2H_2 + O_2 \rightarrow 2H_2O

6\text{ mol}

H_2

and

2\text{ mol}

O_2

react. How many moles of the excess reactant remain?

Example 13

medium

N_2 + 3H_2 \rightarrow 2NH_3

2\text{ mol}

N_2

and

9\text{ mol}

H_2

react. How much excess reactant remains?

Example 14

medium

CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O

3\text{ mol}

CH_4

and

4\text{ mol}

O_2

react. Which is excess and how much remains?

Example 15

medium

A + 2B \rightarrow C

, you have

5\text{ mol}

A

and

8\text{ mol}

B

. How much of the excess reactant remains?

Example 16

medium

2Al + 3Cl_2 \rightarrow 2AlCl_3

4\text{ mol}

Al

and

9\text{ mol}

Cl_2

react. How many moles of excess reactant remain?

Example 17

medium

A + B \rightarrow C

(

1:1

30\text{ g}

A

(

M=30\text{ g/mol}

) and

50\text{ g}

B

(

M=25\text{ g/mol}

) react. Mass of excess

B

remaining?

Example 18

challenge

2H_2 + O_2 \rightarrow 2H_2O

8\text{ g}

H_2

(

M=2

) and

96\text{ g}

O_2

(

M=32

) react. How many grams of excess reactant remain?

Example 19

challenge

N_2 + 3H_2 \rightarrow 2NH_3

, after reacting

1\text{ mol}

N_2

with

4\text{ mol}

H_2

, how many total moles of gas (excess + product) are present at completion?

Example 20

challenge

A + 3B \rightarrow 2C

, mixing

x

mol

A

with

7\text{ mol}

B

leaves

1\text{ mol}

B

in excess. Find

x

(assume

A

is limiting).

Example 21

easy

A + B \rightarrow C

(1:1),

3\text{ mol}

A

and

7\text{ mol}

B

react. Which is the excess reactant?

Example 22

easy

Zn + 2HCl \rightarrow ZnCl_2 + H_2

1\text{ mol}

Zn

reacts with

3\text{ mol}

HCl

. Which is in excess?

Example 23

easy

A reaction needs reactants in a

1:2

ratio. You mix

2\text{ mol}

of each. Which is in excess?

Example 24

medium

2Mg + O_2 \rightarrow 2MgO

6\text{ mol}

Mg

reacts with

4\text{ mol}

O_2

. How many moles of the excess reactant remain after the reaction?

Example 25

medium

2KClO_3 \rightarrow 2KCl + 3O_2

... this is a decomposition, no excess possible. Instead, for

Fe + S \rightarrow FeS

(1:1),

5\text{ mol}

Fe

and

3\text{ mol}

S

react. Excess remaining?

Example 26

medium

3Fe + 2O_2 \rightarrow Fe_3O_4

9\text{ mol}

Fe

reacts with

8\text{ mol}

O_2

. How many moles of excess reactant remain?

Example 27

medium

CaCO_3 + 2HCl \rightarrow CaCl_2 + CO_2 + H_2O

2\text{ mol}

CaCO_3

and

3\text{ mol}

HCl

react. How many moles of excess reactant remain?

Example 28

medium

4Fe + 3O_2 \rightarrow 2Fe_2O_3

8\text{ mol}

Fe

and

9\text{ mol}

O_2

react. Which is the excess reactant and by how much?

Example 29

medium

S + O_2 \rightarrow SO_2

32\text{ g}

S

(

M=32

) and

64\text{ g}

O_2

(

M=32

) react. How many grams of excess reactant remain?

Example 30

medium

2H_2 + O_2 \rightarrow 2H_2O

10\text{ mol}

H_2

and

4\text{ mol}

O_2

react. How many moles of

H_2

remain at completion?

Example 31

medium

2C + O_2 \rightarrow 2CO

6\text{ mol}

C

and

4\text{ mol}

O_2

react. How many moles of excess reactant remain?

Example 32

medium

Cu + 2AgNO_3 \rightarrow Cu(NO_3)_2 + 2Ag

0.5\text{ mol}

Cu

and

0.8\text{ mol}

AgNO_3

react. Which is excess and how much remains?

Example 33

hard

4NH_3 + 5O_2 \rightarrow 4NO + 6H_2O

20\text{ g}

NH_3

(

M=17

) and

40\text{ g}

O_2

(

M=32

) react. How many grams of excess reactant remain?

Example 34

hard

N_2 + 3H_2 \rightarrow 2NH_3

14\text{ g}

N_2

(

M=28

) and

6\text{ g}

H_2

(

M=2

) react. How many grams of excess reactant remain?

Example 35

hard

C_3H_8 + 5O_2 \rightarrow 3CO_2 + 4H_2O

2\text{ mol}

C_3H_8

and

8\text{ mol}

O_2

react. How many moles of excess reactant remain?

Example 36

hard

2Al + 6HCl \rightarrow 2AlCl_3 + 3H_2

5.4\text{ g}

Al

(

M=27

) and

0.5\text{ mol}

HCl

react. Find moles of excess reactant remaining.

Example 37

hard

2H_2 + O_2 \rightarrow 2H_2O

, after combustion you find

0.8\text{ mol}

of one reactant left and

0.6\text{ mol}

H_2O

produced. Identify the excess reactant and the original amount supplied.

Example 38

hard

CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O

16\text{ g}

CH_4

(

M=16

) and

48\text{ g}

O_2

(

M=32

) react. How many grams of excess reactant remain?

Example 39

challenge

A mixture of

0.3\text{ mol}

H_2

and

0.3\text{ mol}

O_2

is sparked. Reaction:

2H_2 + O_2 \rightarrow 2H_2O

. After reaction, total moles of gas (assuming water is liquid)?

← Back to Excess Reactant Practice Mode

Related Concepts

Limiting Reactant Theoretical Yield

Background Knowledge

These ideas may be useful before you work through the harder examples.

limiting reactant