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When two chemicals are combined, the amount of product is stoichiometrically linked to the chemical that is completely consumed. This chemical is referred to as the “limiting reagent”. If any of the other starting material is left over, it is said to be “in excess”. The purpose of this worksheet is to give you practice calculating how much excess remains after a reaction.

 N2  +  3 H2   à   2 NH3 molar mass (g/mole) N2 28.0 H2 2.02 NH3 17.0

Example:

25.0 g of N2 and 10.0 g of H2 are reacted. The 25.0 g of N2 is completely consumed. How many grams of excess H2 is left over after the reaction?

Solution:

Determine how many grams of the hydrogen is used by solving a stoichiometry problem.

Step 1: In using 25.0 g of N2, 5.41 g of H2 was consumed.

Step 2:

Now, subtract the amount of H2 consumed from the amount of H2 that was available.

10.0 g H2 – 5.41 g H2 =  4.6 g H2

There will be 4.6 g of H2 left over. This is the amount of excess.

General Questions
 15.0 g of N2 and 20.0 g of H2 are combined. The N2 is completely consumed. How many grams of excess H2 remain?
 1

 20.0 g N2 and 4.00 g of H2 react. The H2 is completely consumed. How many grams of excess N2 remain after the reaction?
 2

 A reaction starts with 25.0 g of H2 and 25.0 g of N2. How many grams of excess H2 will remain?
 3

 90.0 g of N2 and 20.0 g of H2 react. The N2 is totally consumed. How many grams of H2 are left over?
 4

 50.0 g of N2 and 150. g of H2 are combined. The H2 disappears completely. How many grams of the N2 will remain?
 5

G Waller

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