Stoichiometry Problems With Example

As we know that in a chemical reaction, we have certain numbers written before the reactant or the product that help us to determine the amount of the reactant or the product present in the reaction. Those numbers in the language of chemistry are known as stoichiometric coefficients, and stoichiometry helps us to understand the relationship developed between the reactant and the product and how it changes or remains the same before or after the reaction.

The common terms that we come across in stoichiometric problems are moles, mass, gram molecular mass etc. Let us understand each term clearly. 

Mole: The SI unit for measuring any substance in chemistry is the mole. It is the standard unit meant for measuring any substance. The abbreviation that we use for a mole is mol. In a mole, 6.02214076*10^23 particles are present. The value of a mole was defined on the basis of the isotope of Carbon 12.

Today, we say that Avogadro’s number is the same as that of a mole. The value for one mole is the same as that of Avogadro’s number. The Avogadro’s number has the same number of particles, i.e. 6.02214076*10^23. The value of 1 mole helps us to convert moles to grams or to calculate molar mass etc. 

Molar mass: As the name suggests, the molar mass of any substance can be calculated by knowing the moles and the mass of the given substance. If we divide the mass of the given substance by the number of moles of the substance, we will obtain the molar mass for that particular substance. 

Gram molecular mass: Gram molecular mass is the name of the mass that is calculated for one mole of the substance in the unit grams. 

Gram molecular mass also helps in the stoichiometric problems and the concepts revolving around it. 

Types of stoichiometry 

There are basically three types of stoichiometry that we deal with in our daily life :

1. Mole

2. Mass

3. Volume

All the problems based on the stoichiometric calculations revolve around these three types of variables. 

4 types of stoichiometry problems

The four common types of stoichiometry problems are : 

1. Mole-mole

2. Mass – Mass

3. Mass -Volume

4. Volume- volume

Mole-mole problems

Q.Al + HCl  gives AlCl3 + H2. How many moles of HCl are required in the above reaction to react with 0.87 moles of Al? 

Solution: The very first step in every problem of stoichiometry is to balance the given chemical equation. 

After balancing the above equation with the correct stoichiometric coefficients, we get the following equation :

2Al+ 4 HCl gives 2AlCl3 + 3 H2

The second step is to find the number of moles for the given substance

As 0.87moles of Al are given, we can use the relation that

1 mole of HCl requires 0.87 moles of Al

3 moles of HCl will require 3* 0.87 moles of Al = 2.6 moles of HCl

Mass–mass problems

Q.On the decomposition of 9.8 g of Al2O3, how many grams of Al can be formed from it? 

Al2O3 gives Al + O2

The first step to deal with the question is to balance both the reactant as well as the product.

The balanced equation we get is: 2 Al2O3 gives 4 Al + 3 O2

Since we know that the mass of the given substances is 9.8 g,  the next step is to find out its mole

The calculated number of moles comes to 0.096 moles of Al2O3. 

Now we’ll find the moles using the ratios

 0.096 mole of Al2O3 gives 2 moles of Al

So, 1 mole would give 0.019 moles of Al 

The calculated mass using the new moles comes to 5.1 g of Al. 

Mass-volume problems

Q. When 20.0g K reacts as per the given reaction, calculate the number of H2 formed in the process 

K+ H2O gives KOH + H2

Solution :

On Balancing the given equation, we get: 2K + 2H2O gives 2KOH + H2

As the given mass is 20.0g of K, we can calculate the number of moles from it 

As 20g of K gives 1 mole of K, then 39 g of K will give 0.513 moles of K. 

We can now calculate the ratios using the moles:

As 0.266 mole H2 give 22.4 L H2

Then 1 mole of H2 gives 0.266 moles of H2

Now we can calculate the volume using the new moles.

0.266 mole of H2 would give 0.266*22.4 = 5.75 L of H2. 

Volume – Volume 

Just as the questions on mass volume stoichiometry are done, the same approach can be used to deal with the questions based on the volume-volume problems. 

However, it is important to use the correct units as well as the balancing of the equation has to be proper. 

Conclusion

The problems based on the stoichiometric problems require good calculation and a positive approach. The questions can be easily handled if the units are kept in mind. 

Every question needs to first have a balanced equation in order to proceed with the question and calculations.