 # Mass calculations for Edexcel A-level Chemistry 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 End of page