Reacting Masses

When beginning to learn the important concepts and knowledge required to engage with the DP Curriculum it is important to have a basic understanding and experience of some of the ideas in each topic. At AIS the majority of students follow the iGCSE Coordinated Science curriculum as a precursor to studying DP chemistry and this site will use some of the objectives from the iGCSE to provide an area of prior learning - allowing those students who would like to refresh their knowledge or for new students to engage wth matter that has become prior to starting this course.

To this end students are expected to have a basic understanding of the C4 Stoichiometry unit

If we know the mass of a compound in a balanced equation can we calculate the mass of any of the compounds we react or produce?

YES!! If we know the mass of any compound in a balanced equation we can calculate the Minimum mass we would need of each reactant or the maximum mass of the products we would make

Take the following example

"How much Sodium Chloride would be produced from 23 grams of Sodium?"

Step 1: Write Down what we know

Mass: 23 ? --> ?

RFM: 23 71 --> 58.5

Mole Ratio: 2 1 --> 2

Step 2: Work out the moles of the compound with known mass

We know that Mass/RFM = Moles

So for Na - 23 / 23 = 1 Mole Sodium

Step 3: Work out the moles of the Substance we are looking for

In this example we are looking for Mass of Sodium Chloride

Using the Molar ratio from the balanced equation we can see that 2 Moles of Sodium = 2 Moles of Sodium Chloride

We only have 1 mole of Sodium (worked out from Step 2)

Therefore we can only make ONE Mole of Sodium Chloride

To work this out:

• (Moles / Mole ratio) (for the known substance (eg Sodium) X Mole Ratio of unknown substance (eg Sodium Chloride)

• this would be (1/2) X 2 = 1 mole Sodium Chloride

Step 4: Use Moles X RFM to work out MASS of unknown substance

We are still looking for the mass of Sodium Chloride

Using Step 1, 2 and 3 we can see that we would make One mole of NaCl from 23 grams of Sodium

As Moles X RFM = Mass

1 X 58.5 = 58.5g

Finally - we have our answer 24 grams of Sodium would yield 58.5 grams of Sodium Chloride