Reaction Stoichiometry: Mass

This article builds up on these two articles:

Concept of Reaction Stoichiometry and Stoichiometric Coefficients (otherwise known as mole ratios).

This article covers how much grams (or any other unit if you know your unit conversion) you need or can get out of a reaction. There are lots of ways to do this, we're going to teach our favourite for it's easy and quick and builds up on what we've learnt.

Relative Formula Mass This is a very important concept, so here's a recap:

The RFM is how heavy a mole of that particular element, compound or molecule weighs.

Do NOT mix up Atomic Weight and Atomic Number! Usually there's a key or legend at the bottom of your Periodic Table, and different Periodic Tables can have a different key. What do each of them mean? Atomic Weight: RAM and also the Molar Mass, which is how heavy 1 mole of that element is, in grams.

Atomic Number: The number of protons in that element. The number of protons determine what that element is. Usually, the Atomic Weight is the larger number, and most accurate tables will give the weight as a decimal point.

So, 1 Mole of Carbon weighs 12.011 grams.

RFM of a compound is basically the sum of all its parts. For example, in H2O, there are 2 hydrogen molecules and 1 oxygen molecule, so find the molar mass of each and add the together. Details are right here.

Reacting Masses

Yep, that's the name you're gonna see in most textbooks. Basically means "how much something will I get if I react a certain amount of this with an excess of that?" and stuff along those lines.

So, let's cut to the chase with something simple, without any equations: If a reaction with 9.6 g of magnesium can produce 48 g of magnesium sulphate in a certain reaction, how many grams of magnesium sulphate can I get if I react 1.2 g of magnesium?

If you haven't noticed already, this is just another simple ratio-type question set in a chemistry context. The two numbers are proportionate, so all we have to do is to multiply by a common factor to both numbers to get an equivalent ratio. Look, you don't have to know all these big words and stuff, you just have to understand how to do it:

9.6 g Mg -> 48 g MgSO4

Scalar Quantity (Common Factor): 1.2/9.6 = 1/8 Another way to understand this is "how much do we multiply 9.6 to get 1.2, so we can get a number to multiply 48 by to get our mystery quantity".

Now multiply 9.6 by 1/8, and 48 by 1/8, to complete the question:

1.2 g Mg -> 6 g MgSO4

Another way to do this is to manipulate the reaction like a ratio, so you can get a simplified ratio and make them all into different equivalent ratios:

9.6:48 simplified is 1:5

Our ratio is 1.2:X = 1:5 , so we multiply 1.2 by 5 to get mystery quantity X, which is 6 grams.

Now a harder one with an equation...

Mg + O2 -> MgO

What mass of oxygen is needed to react with 12 g of magnesium? DON'T PANIC! Understand how it's done, take it step by step, and you'll never be stuck again. 1) Balance the Equation

Always do this first. ALWAYS. You need the mole ratios, and it's a good practice.

Here's the balanced equation:

2 Mg + O2 -> 2MgO

2) Find out the mole ratio as required by the question

How many moles of oxygen is needed to react with a certain mole(s) of magnesium?

Answer: Look at the equation:

The ratio is 2 : 1

meaning

2 moles of magnesium is needed to react with 1 mole of oxygen

3) Find out the Molar Mass needed for the reaction

Basically, you convert the mole ratio into a mass ratio through converting the moles into mass through the RFM. Seems complicated, but it isn't.

2 mol of Mg : 1 1 mol of O2