# Basic stoichiometry problems

Stoichiometry can be a tricky concept to grasp – but it shouldn’t be. The reason everyone gets stuck on it is because they treat the concept of a mole as a new concept. The mole is a unit of measurement used by chemists to communicate the quantity of chemicals they have. It is no more complicated then understanding eggs! When you go to the shops you say you bought a dozen eggs, rarely do you hear someone say they bought 12 eggs – everyone just knows that is the case. That’s why we chemists talk about moles instead of saying how many individual molecules we have. Don’t worry, follow our worked examples and try and mark your answers to our basic stoichiometry problems and we promise you will smash stoichiometry!

So referring to a one mole solution is no more conceptually different to referring to a dozen eggs. 1 mole of a certain chemical, means you have 6.022 × 10²³ molecules of that chemical!

A mole is to a dozen as × 10²³ molecules are to 12 eggs.

But where does 6.022 × 10²³ come from?

Avogadro’s constant is based on the number of atoms or molecules in one mole of a substance. So essentially if you have 1 mole which is 12 g of the chemical compound, carbon-12, you will have 6.022 * 10^23 atoms

In chemistry, a mole is a unit of measurement for amount of substance. It is defined as the amount of a substance that contains the same number of entities (such as atoms, molecules, or ions) as there are in 12 grams of pure carbon-12. The mole is used in chemical reactions to determine the number of atoms, molecules or ions of a substance. The symbol for mole is ‘mol’. It is an important concept in many areas of chemistry, including stoichiometry and chemical thermodynamics.

## Stoichiometry

The concept of moles is foundational to stoichiometry. Stoichiometry can be hard to pick up at first, but will become second nature with enough practice. Most students reach the stoicheometry section and begin trying to momorize formula like the concentration (M) = the number of moles (n) divided by the volume in litres. I never liked this formula, I preferred to understand the concept rather then waste time blindly memorizing things.

So how do I remember it? I think of cordial. You mix the syrup

In stoicheometry I saved my time by using one method, it is the unit method. Essentially when you use this method a 4 mark HSC calculation question quickly turns into a trivial one line solution. This unit method is like learning to ride a bike, once you get it you won’t be able to believe you struggled with it.

Stoichiometry originally meant to measure elements. Stoikhein is Greek for element and metron for measure.

Stoichiometry is important because it allows us to quantify the relationships between the amounts of reactants and products in a chemical reaction. It helps us to predict the amount of product that can be obtained from a given amount of reactant, and to determine the amount of reactant required to produce a desired amount of product. In addition, stoichiometry is used in many fields such as materials science, pharmaceuticals, and environmental science to control and optimize the yield of reactions and processes.(1)

Stoichiometry is really just the mathematics behind chemistry! These worked examples will help you with the basic stoichiometry problems below.

Example Calculation 1.

We can use the unit method for doing our stoichiometry calculations. You have your units cancel. Let’s have a look at this question:

You have a 3 L solution of Hydrochloric acid with a concentration of 4 mol/L. How many moles of hydrochloric acid are there?

See we multiply 3 L by 4 mol/ L since the L will cancel out since it is on the denominator, hence leaving mol remaining.

Example calculation 2.

You have a 2 L solution of sodium hydroxide with a concentration of 3 mol/L. How many molecules are there?
Note: Avagandro’s constant = 6.022  × 1023 molecules/ mol

Example calculation 3.

You have a solution of hydrochloric acid with a concentration of 2 mol/L. You want 3 mol of hydrochloric acid, what volume do you require to get this?

## Basic stoichiometry problems.

Question 1.

You have a 500 mL solution of lithium hydride with a concentration of 8 mol/L. How many moles of hydrochloric acid are there?

Question 2.

You have a 8 L solution of sodium hydroxide with a concentration of 2 mol/L. How many molecules are there?

Question 3.

You have a solution of magnesium hydroxide with a concentration of 1.5 M. You want 3 mol of hydrochloric acid, what volume do you require to get this?