# WAEC Chemistry Revision: Introduction to Stoichiometry

Stoichiometry is the branch of chemistry which deals with the calculation of quantities of any substance in a chemical reaction. There are 4 must follow steps to better accuracy in Stoichiometry. Step 1 and 2 are unique, understanding how to get them right is the first stage to becoming a successful Chemistry student.

Step 1: Balancing of chemical equations

Step 2: factorization of the coefficients in the balanced chemical equations.

The balanced chemical equation gives the ratio of formula units of all the substance in the chemical reaction. The ratio of the coefficients in the balanced chemical equations can be used as conversion factors for solving Stoichiometry problems.

Example 1: write all the possible factors from the coefficients in the following balanced chemical equations in the image above.

Example 2: write all the possible factors from the coefficients in the balanced chemical equation, in the image below.

The ratio that is used to solve a particular Stoichiometry problem, will have the substance for which the number of moles is given in the denominators, and the substance in which the number of moles is desired as the numerator.

Example 3; Calculate the number of moles of aluminum atoms, that will react with 3.18 mol of oxygen molecules to form aluminum oxide. And the number of moles of aluminum oxide produced.

Step 1: write and balance the chemical equation for the reaction of Aluminum and Oxygen.

step 2 : write all the possible factors from the coefficients in the balanced chemical equation

Step 3: now go for substance for which the number of moles is given in the denominators, 3.18 moles of oxygen has been given in the question. That is why have circled both factors necessary to the solution of the problem.

step 4: multiple the factors by the amount of oxygen given in the question.

Conclusion; 4.24moles of Aluminum will burn in 3.18mole oxygen gas to produce 2.12 moles of Aluminum oxide.

Example 4: Calculate the number of moles of Hydrogen gas that can be produced by reaction of 0.750 mole of hydrochloric acid, HCl, with calcium metal.

Step 1; write and balance the chemical equation all substance involved in reaction

Step 2; write all the coefficients factors possible from the balanced chemical equation

Step 3; now go for substance for which the number of moles is given in the denominators, all denominators of HCl acid, has been circled in the diagram below. Because we are given 0.750 mole of hydrochloric acid in the question,.

Also, The HCl denominator carrying Hydrogen gas as the numerator has been marked. Because we are asked to find the moles of hydrogen gas produced.

Step 4 ; multiple the marked factor by the amount of HCl acid given in the question

Conclusion; 0.750 mole of hydrochloric acid will produce 0.375 mole of Hydrogen, when combined with calcium.

Example 4; Calculate the number of moles of aqueous NaOH that must react completely with aqueous H₂SO₄ to produce 1.24 mole of Na₂SO₄.

Step 1: the question is a neutralization reaction, the other three compounds have been given in the question, water is the only substance left to be included. Include H₂O, and write the balanced equation. If you’re correct, yours should look like mine,

Step 2;write all the coefficients factors possible from the balanced neutralization reaction, above.

Step 3now go for substance for which the number of moles is given in the denominators, all denominators of Na₂SO₄, has been circled in the diagram below. Because we are told that 1.24 mole of Na₂SO₄ has been produced in the reaction, said the question,.

Also, The Na₂SO₄ denominator carrying NaOH aqueous as the numerator has been marked. Because we are asked to find the number moles of aqueous NaOH that must complete the reaction.

Step 4 ; multiple the marked factor by the amount 1.24 mole of Na₂SO₄ given in the question

Conclusion; 2.48 moles of aqueous NaOH will be required to completely neutralize aqueous H₂SO₄ to produce 1.24 mole of Na₂SO₄ salt.

Practice Question 1: A sample of 0.1712 mole of solid KClO₃ is heated gently for a time, and 0.1146 mole of the compound decomposes. Calculate the number of moles of oxygen produced.

Practice Question 2: In a certain reaction, 0.225 mole of H₂ gas reacts partially with N₂ gas to yield gaseous NH₃ . if 0.033 mole of H₂ remains, after the reaction is stopped, how many moles of N₂ is used up?

Hint: follow the 4 systematic steps as used in the above Sample problems.

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