Question

There are 1000 P₄ molecules and 500 S₈ molecules in a container. These are made to react and form P₂S₅ molecules. How many molecules of the excess reactant would remain unreacted in the reaction vessel? Use factor label method for your calculations.

Select one:

a. 800

b. 2400

c. 600

d. 1200

Answer 1

We can solve given problem in following steps.

Step 1 : Writing a balanced chemical equation.

Consider balanced reaction, 4 P ₄ + 5 S ₈ 8 P₂S₅

Step 2 : Determination of Limiting reactant.

According to reaction, 4 mol P ₄ reacts with 5 mol S ₈.

4 molecules P ₄ reacts with 5 molecules S ₈.

Molecules of S ₈ required to react with 1000 molecules of  P ₄ =1000 molecules P ₄  ( 5 molecules S ₈. / 4 molecules P ₄ )

= 1250 molecules S ₈

Provided molecules S ₈ are less than required. Hence, it is limiting reactant.

Limiting reactant : S ₈

Step 3 : Calculation of amount of excess reactant consumed in the reaction.

Now, calculate molecules of  P ₄ required to react with 500 Molecules of S ₈.

molecules of  P ₄ required to react with 500 Molecules of S ₈ = 500 Molecules of S ₈ (  4 molecules P ₄ / 5 molecules S ₈.)

= 400 molecules P ₄

Step 4 : Calculation of amount of excess reactant left after completion of reaction.

Excess molecules P ₄ = 1000 molecules - 400 molecules = 600 molecules.

ANSWER : C) 600