Limiting Reactant Formula

In any chemical reaction, the limiting reactant(or reagent) is a substance which is totally consumed when the chemical reaction is complete. This reagent limits the quantity of the product produced and the reaction cannot continue without it. In a precipitation reaction, the incomplete precipitation gives the idea of limiting reagent. In dissolution reaction, the clear saturated or unsaturated solution gives the limiting reactant. In case of gaseous reaction, the limiting reagent is found by analyzing the mixture of gases that are formed.

The procedure to determine the limiting reagent is as follows

Balance the chemical equation if not balanced.

Find out the number of moles of every reactant in the reaction.

Calculate the amount of products that can be produced from the complete reaction.

Determine which reactant would produce the minimum quantity of product, and that is the        limiting reagent.

Example 1

2.3 g of sodium metal is transferred to a 3L flask filled with chlorine gas. Determine the limiting reagent and amount of excess reagent present if the mass of Na = 23 and Cl = 35.5.

Solution

The Balanced equation is

$2Na&space;+&space;Cl_{2}\rightarrow&space;2NaCl$

2.3 g of sodium metal = 2.3 / 23 = 0.1 mole

It requires 1 x 0.1/2 = 0.05 moles of chlorine

1 mole of chlorine gas = 22.4 L at STP

Therefore, 3 L = 3/22.4 = 0.1339 moles

Hence, Sodium metal is the limiting reactant.

The amount of Cl2 in excess is 0.1339 – 0.05 moles

= 0.0839 moles occupy 0.0839 x 22.4 / 1

= 1.88 L approximately.

Example 2

An aluminium wire weighs 54g and is dipped in 2 L of 1M sulfuric acid. Determine which is the limiting reactant. Atomic mass of Al is 27.

Solution

The Balanced equation is

$2Al+3H_{2}SO_{4}\rightarrow&space;Al_{2}(SO_{4})_{3})+3H_{2}$

The equation suggests that 2 moles of Al require 3 moles of $H_{2}SO_{4}$

54 g of Al = 54/27 = 2 moles which require 3 moles of $H_{2}SO_{4}$

We have 2L of 1M $H_{2}SO_{4}$ which equals 2 moles

Therefore, Sulfuric acid is the limiting reactant.