When can we use the molar ratio?

Does the equation have to be in equilibrium with the double arrows or can we also use it for single arrows?

1 Answer
Stefan V.
Feb 24, 2017

Anytime you're looking at a balanced chemical equation.

Explanation:

The mole ratios that exist between the chemical species that take part in a reaction are simply the stoichiometric coefficients needed to balance the chemical equation that describes said reaction.

Mind you, it doesn't matter if the reaction goes to completion or if you're dealing with an equilibrium reaction, but it does matter if the chemical equation is balanced or not.

More specifically, you can only use mole ratios when the chemical equation is balanced.

Let's take, for example, the reaction

CH4(g)+2O2(g)→CO2(g)+2H2O(l)

Notice that the reaction consumes 2 moles of oxygen gas for every 1 mole of methane that undergoes combustion and produces 1 mole of carbon dioxide and 2 moles of water.

These mole ratios tell you that regardless of how many moles of one reactant you have, there will always be a 1:2 mole ratio between them.

Similarly, the reaction will always produce twice as many moles of water than of carbon dioxide because the two reactants are in a 2:1 mole ratio.

So, if 7 moles of methane react, you will have

7moles CH4⋅2.moles O21mole CH4=14 moles O2→ needed

7moles CH4⋅1 mole CO21mole CH4=7 moles CO2→ produced

7moles CH4⋅2.moles H2O1mole CH4=14 moles H2O→ produced

I recommend checking out this excellent video on how to use mole ratios to solve stoichiometry problems

Related questions
See all questions in Mole Ratios
Impact of this question
8131 views around the world
You can reuse this answer
Creative Commons License