# Limiting Reagent in Reactions - How to Identify, Calculate, Examples

In a reaction, one reactant can be finished earlier while other reactants are still available to react in the reaction medium. That finished reactant is defined as limiting reagent and reaction will be immediately stopped when this limiting reagent is finished. In this tutorial, we will discuss followings to get a good knowledge about limiting reagent in chemistry.

• Stoichiometric ratio, amount of reactants and selection of limiting reagent
• Determine limiting reagent in a solution based calculations
• Limiting reagent and excess reagent

## Stoichiometric ratio, amount of reactants and selection of limiting reagent

Reactants react with each other in a different ratios in different reactions and several examples for this case are explained below.

### Stoichiometric ratio

#### NaOH + HCl → NaCl + H2O

• 1 mol of Sodium hydroxide requires 1 mol of hydrochloric acid for a complete reaction because in the balanced chemical equation, the ratio between sodium hydroxide and hydrochloric acid is 1:1

#### H2SO4 + 2NaOH → Na2SO4 + 2H2O

• 1 mol of sulfuric acid requires 2 mol of sodium hydroxide for a complete reaction because in the balanced chemical equation, the ratio between sulfuric acid and sodium hydroxide is 1:2

### Amount of reactants and selection of limiting reagent

In the beginning of a reaction, there is a certain amounts (mol) of reactants and let's discuss with an example.

#### Example: 1 mol of sodium hydroxide and 0.9 mol of hydrochloric acid

As an example, we can consider sodium hydroxide ad hydrochloric acid reaction. In the beginning of reaction, there are 1 mol of sodium hydroxide and 0.9 mol of hydrochloric acid. We have seen that sodium hydroxide and hydrochloric acid react with 1:1 ratio according to the stoichiometric ratio of balanced equation.

That means, for a complete reaction between sodium hydroxide and hydrochloric acid, same amount is required. But, in this example, there is only 0.9 moles of hydrochloric acid is available to react with 1 mol of sodium hydroxide. So, total amount of hydrochloric acid is not enough is react with total sodium hydroxide amount. Therefore, total hydrochloric acid amount is reacted and consumed and some sodium hydroxide remain in the final solution.

Calculation steps can be shown as below for better explanation.

#### Identify what is the limiting reagent when 0.1 mol dm-3 KOH 25 cm3 is mixed with 0.2 mol dm-3 HNO3 12 cm3?

To decide the limiting reagent, we should know the amount of reactants. Because, both are aqueous solutions, we can find the amount by given data (concentration and volume).

#### Mixed amount of KOH

• Mixed amount of KOH = 0.1 mol dm-3 * 25 * 10-3
• Mixed amount of KOH = 0.0025 mol

#### Mixed amount of HNO3

• Mixed amount of HNO3 = 0.2 mol dm-3 * 12 * 10-3
• Mixed amount of HNO3 = 0.0024 mol

Because, now we know the initial amounts of reactants, we can write balanced equation to find the what is the limiting reagent.

#### KOH + HNO3 → KNO3 + H2O

For a complete reaction, similar amounts of KOH and HNO3 are required. Therefore, for 0.0025 mol of KOH, 0.0025 mol HNO3 is required. But, in this case, there is only 0.0024 mol of HNO3.

Therefore, total amount of HNO3 is not enough to react with 0.0025 mol of KOH. Therefore, HNO3 becomes the limiting reagent.

## Identify limiting reagent in preparing buffer solution with ammonia and hydrochloric acid

You were given 40 cm3 of 0.1 M ammonia solution and you have added 10 cm3 of 0.1 M HCl solution. Find which reagent will be the limiting reagent?

Ammonia and hydrochloric acid reacts with each other and form ammonium chloride as the product. Ammonium ion shows acidic characteristics. Therefore, if both ammonia and ammonium chloride exist in a considerable concentration after the reaction, final solution can be a buffer solution.

### NH3 + HCl → NH4Cl

Then, we should calculate how much HCl and NH3 are consumed during the final reaction.

• Initial amount of ammonia = 0.1 M * 0.040 dm3
• Initial amount of ammonia = 0.004 mol

• Added amount of HCl = 0.1 M * 0.01 dm3
• Added amount of HCl = 0.001 mol
• Formed amount of ammonium chloride = 0.001 mol

Both reactants react with each other in 1:1 ratio. Because, initial ammonia amount is higher than added amount of HCl, ammonia remains in the final solution with ammonium chloride. Therefore, HCl becomes the limiting reagent in this reaction.

#### Related Tutorials

pH to pOH calculator Calculate pH of sulfuric acid pOH of KOH Solution pOH of aqueous HCl solution Online pH calculator for weak acids

### Lewis structures of molecules

P2O5 lewis structure OH- lewis structure Ammonium ion (NH4+) lewis structure H2CO3 lewis structure