How do you determine how much of the excess reactant is left over? Also, how do you determine how much MORE of the limiting reagent would you need to use up the excess?
Once you have identified the limiting reactant, you calculate how much of the other reactant it must have reacted with and subtract from the original amount.
A Sandwich-Making Analogy
This video from Noel Pauller uses the analogy of making sandwiches.
The general problem
Given the chemical equation and the masses of reactants, determine the mass of excess reactant and the mass of the limiting reactant required to use up the excess.
A specific problem
A 2.00 g sample of ammonia reacts with 4.00 g of oxygen according to the equation
How much excess reactant remains after the reaction has stopped? How much more of the limiting reactant would you need to use up the excess?
- Write the chemical equation.
- Calculate the moles of product from the first reactant.
- Calculate the moles of product from the second reactant.
- Identify the limiting reactant and the excess reactant.
- Calculate the mass of excess reactant used up.
- Calculate the mass of unused excess reactant.
- Calculate the mass of limiting reactant needed to react with the unused excess reactant.
1. Balanced equation
2. Moles of
Convert grams of
The molar mass of
3. Moles of
The molar mass of
We know from the balanced equation that the molar ratio is
4. Identify limiting and excess reactants
5. Calculate the mass of excess reactant used up.
Use the molar ratio from the equation to convert moles of
6. Calculate the mass of unused excess reactant.
We started with 2.00 g of
7. Calculate the mass of limiting reactant needed to react with the leftover excess reactant.
It takes 0.70 g of
Here is another example...