Stoichiometry Life’s answer to the eternal question “how many sandwiches can I make from the leftovers in my fridge?” (one, if you do it right)

StoichiometryLife’s answer to the eternal question “how many sandwiches can I make from the leftovers in my fridge?”

(one, if you do it right)

Stuffy DefinitionStoichiometry: the relationship between amounts of a compound/element in a chemical reaction. Usually expressed in terms of ratios.

Useful DefinitionStoichiometry: the relationship between amounts of a compound/element in a chemical reaction. Usually expressed in terms of ratios.

Stoichiometry: “how much of each of these things do I need?”

Example Reaction2Mg (s) + O2 (g) 2MgO (s)

Our starting point is always a complete, balanced, chemical reaction.

When you did this lab, you might have started with 1.00 grams of Mg, and found that you got 1.656 grams of magnesium oxide. Therefore, we get the following results:

2Mg (s) + O2 (g) 2MgO (s) 1.00g 0.658g 1.658g

Basic conservation of mass here; if we get an extra 0.658g out, if must have come from the oxygen.

Unfortunately, these numbers seem…rather arbitrary? There isn’t really any sort of obvious relationship between them.

But What If We Use Moles?Reaction Equation: 2Mg (s) + O2 (g) 2MgO (s) Mass: 1.00g 0.658g 1.658gMoles: 0.041 0.0205 0.041

Notice something about those numbers?

But What If We Use Moles?Reaction Equation: 2Mg (s) + O2 (g) 2MgO (s) Mass: 1.00g 0.658g 1.658gMoles: 0.041 0.0205 0.041

Notice something about those numbers?

Moles of Mg and moles of MgO are the same, and both are exactly double the moles of O2…

…which is exactly the same as the coefficients from the balancing.

That’s What the Balancing IsReaction Equation: 2Mg (s) + O2 (g) 2MgO (s) Mass: 1.00g 0.658g 1.658gMoles: 0.041 0.0205 0.041

The balancing is actually the thing that tells you how many of reactant #1 (Mg) combine with how many of reactant #2 (O2) to make how many of the product (MgO).

And how many in chemistry is always measured in moles.

An AnalogyRecipe:

2 Slices of bread4 tomato slices5 strips of bacon1 leaf of lettuce

In chemistry form, we could write this as:

2Bread + 4Tomato + 5Bacon + 1Lettuce 1BLT

Note: this reaction equation doesn’t tell you how much stuff you have, just the proportions of each needed to combine to make a sandwich.

An Analogy

Important!Did the 5 in front of bacon in the recipe change how much bacon I have?

Of course not. I have 15 strips. (well, I don’t really have any…vegetarian and whatnot)

The 5 simply is part of the stoichiometry of making a BLT: the proportions needed to make one according to the recipe.

Same goes for chemistry: the number in the reaction equation doesn’t change how many moles you have; it just tells you the proportions of other stuff you need.

So Back To Magnesium

So Back To Magnesium

So Back To Magnesium

So Back To Magnesium

Result

Molecular weight Molecular weightProportion of coefficients

Summary• A balanced reaction equation gives you the proportions

things react in.

• The moles that react will always be in that proportion.

• Ba(CON)2 is a theoretically possible compound, though no chemist would actually write it in that order, and it doesn’t appear to be made/used for anything.