How to Understand Stoichometry

Tammy Duong

What is Stoichometry?

The dictionary version says "the relationship between the relative quantities of substances taking part in a reaction or forming a compound, typically a ratio of whole integers". In other and less fancy words, it's creating a chemical compound or reaction while maintaining the same ratio of elements throughout.

Let's Start with the Basics

Given Reaction: Iron(II) Sulfate + Barium Nitrate

It's a Double Replacement type of reaction. Remember to switch metal with metal OR non-metal with non-metal.

Keep the equation balanced on both sides, don't forget the charges and include the states of matter! (States of matter should be either aq, s, or l)

Balanced Equation (with states of matter):

Fe (SO4) aq + Ba (NO3)2 aq -----> Fe (NO3­)2 aq + Ba (SO)4 s

Get to Know the Elements


Let's Talk about Conversions and How To Do It

Important: Remember to keep the mole ratio,the conversion factor for any two reactants/ products in a chemical reaction, consistent and the same throughout the process of solving!

Okay, so conversions with two different elements... It seems difficult but it's not. Keep in mind what you are solving for, the units, and of course the mole ratio.

Now how to do it, well you've already learned how to do mole to mole conversion which is simple enough. Given an amount of moles from the question, you use the coefficients of your balanced equation and place it in the right spot. Multiply across the top and divide by the bottom. If you don't remember there'll be a visual example below. So that's mole to mole but then there's mass to mass conversion. This process is much longer but it's actually simple. Explaining it in words will just confuse everyone so look at the picture!

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Limiting and Excess Reactants

Have you ever made S'mores and had only marshmallows and graham crackers left? Well this actually has something to do with stoichiometry! There was only a certain amount of s'mores that you could make WHILE keeping the SAME RATIO of graham crackers to marshmallows to chocolate. The chocolate is what we call the "limiting reactant. It limited the amount of s'mores you could make and in a actual chemical reaction, the chocolate would be the first to be used up. What's leftover is the marshmallows and graham crackers called "excess reactants". So how to we calculate the excess and limiting reactant? Check out the pictures below!

Theoretical and Percent Yield

Alright one last thing through the journey of stoichiometry, theoretical and percent yield. So remember your limiting reactant? Well, that's your theoretical yield! It's what in theory you should have left after the chemical reaction. Percent yield is what you find when you divide the actual yield (normally given in the question) and the theoretical yield times 100.