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Mass calculations for AQA A-level Chemistry
This page covers the following topics:
1. Percentage mass
2. Theoretical mass
3. Atom economy
Percentage mass is a way of representing the percentage of an element in a compound. Mass percentage is calculated by the mass of the element divided by the total mass of the compound.
Due to incomplete reactions, when experiments are carried out there is not a 100% yield of products. The theoretical yield is the amount of product you could produce theoretically. The percentage yield is a measure of the amountthe actual yield you get from an experiment.
Atoms are not gained or lost in a chemical reaction, some atoms from the reactants end up in the product, and some in by-products. The atom economy of a reaction expresses how efficiently a reaction makes use of reactant atoms, i.e. how much product is creating in comparasion to by-product. Atom economy is a percentage, thus the highest it can possibly be is 100%.
1
What is the definition of atom economy?
A measure of the amount of starting elements/compounds that end up as useful products.
2
3 g of C₃H₇OH has reacted with an excess of CH₃COOH (Ar = 60 g/mol) to give 1.8 g of C₃H₇OOCCH₃ (Ar = 102 g/mol) and some water, what is the actual yield of this reaction?
Actual yield is the experimental mass of the product.
1.8 g
3
What is the theoretical yield?
The maximum possible mass of product you could produce from a reaction
4
What is the percentage mass of oxygen in CO₂?
(16 x 2) / 44 = 72.73%
5
What is the actual yield?
The actual yield is the final mass of product produced in a reaction.
the actual mass of product yielded in a reaction
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