Mole Concept Questions on Excess and Limiting Reagent

In this post, let's try a question on excess and limiting reagent. This type of questions come from the chapter mole concept and stoichiometry.

Question

1.0 mole of Cu₃FeS₃ and 1.0 mole of O₂ are allowed to react according to the following equation:

2Cu₃FeS₃(s) + 7O₂(g)  --> 6Cu (s) + 2FeO(s) + 6SO₂ (g)

Which of the following statement is true?

A. 0.286 mol of Cu₃FeS₃ is in excess

B. 0.714 mol of Cu₃FeS₃ is in excess

C. 5.0 mol of O₂ is in excess

D. no reagent is in excess

Answer: B

Explanation

From the equation, mole ratio of Cu₃FeS₃:O₂  is 2:7.

From the above equation, we know that we need more (moles) of  Cu₃FeS₃ than O₂ . Since we use the same amount of both, but we need more O₂ for the reaction, this means Cu₃FeS3 is in excess (or O₂ is the limiting reagent). Hence, options C and D are incorrect.

Now, we know is in excess, hence, all of it will react as it is the limiting reagent. 

2 mol of  Cu₃FeS₃: 7 mol of:O₂ 

2/7  mol of  Cu₃FeS₃: 1 mol of:O₂ 

Since 1 mol of Cu₃FeS₃ is used,  1 - 2/7 = 5/7 mol of Cu₃FeS₃ is in excess.

5/7 = 0.714 mol (3 decimal places)

Hence, 0.714 mol of Cu₃FeS₃ is in excess. The correct answer option is B.