Question

How many moles of C2H2 are needed to react completely with 84.0 mol O2?

Answer 1

Answer: 33.6 moles

Explanation:

According to the law of conservation of mass, mass can neither be created nor be destroyed. Thus the mass of products has to be equal to the mass of reactants. The number of atoms of each element has to be same on reactant and product side. Thus chemical equations are balanced.

Combustion of ethylene is represented by following balanced equation:

$2C_2H_2+5O_2\rightarrow 4CO_2+2H_2O$

According to stoichiometry:

5 moles of $O_2$ react completely with = 2 moles of $C_2H_2$  

Thus 84.0 moles of $O_2$ react completely with =$\frac{2}{5}\times 84.0=33.6$ moles of $C_2H_2$  

Thus 33.6 moles of $C_2H_2$ are needed to react completely with 84.0 mol $O_2$

Answer 2

33.6 moles are needed to completely react with 84.0 moles of O2