I am sure it comes as no surprise that here we’re going to look at some rules and equations to guide you through quantitative chemistry.
So let’s first look at some principles to stick to when thinking about equations.
A balanced symbol equation is not only able to tell us about the products and reactants of a chemical reaction, but also about the amount of each substance reacting.
A reversible reaction is when reactants react to form products and the products can react to make the original reactants again.
In other words, a reversible reaction can go in both the forward and reverse directions and so it is represented by two arrows. This can sometimes be a reason why the maximum possible yield is not obtained in a chemical reaction.
It is also important to remember that not all atoms have a whole number as their relative atomic mass, so if you don’t get a whole number in a calculation don’t automatically think you’ve got it wrong!
The relative atomic mass is the average mass of the different isotopes.
For example a carbon atom with six neutrons and six protons has a mass of 12 units (because the relative mass of neutrons and protons is one).
We use this as a standard to calculate the relative atomic mass of other atoms. But not only does the relative atomic mass take into account the presence of isotopes but also their relative proportions. So an atom cannot have fractions of protons and neutrons in its nucleus, but the relative atomic mass might not be a whole number.
Next up, yields.
We can calculate the maximum yield when we know the balanced equation and the relative formula masses of the reactants and products. However, the maximum yield is often not obtained.
This could be because some product has been left behind in the reaction apparatus or the reactants may not be pure.
Often, the actual yield is expressed as a percentage yield.
Percentage yield = (amount of product produced / maximum amount of product possible) x 100%
Lastly, how to calculate the percentage of an element in a compound:
First write down the formula of the compound. Then calculate the relative formula mass of the compound.
We’re going to use the example of ammonia. So the relative formula mass of ammonia = 14 + (3 x 1) = 17.
Divide the mass of the element you are investigating by the relative formula mass of the compound = 14/17 = 0.824.
And then multiply by 100 to get a percentage = 82.4%
See that wasn’t so bad. Now try doing some of your own calculations using different elements!