 # Mass calculations for OCR GCSE Chemistry 1. Conservation of mass
2. Relative formula mass
3. Percentage mass
4. Theoretical mass
5. Atom economy

The principle of mass conservation states that for any system closed to any tranfer of matter or energy, the mass of they system must remain constant, meaning no quantity of atoms can be added or taken away. This means there must be equal atoms of each element on either side of an equation. This is true only in an enclosed system. The relative formula mass of a compound is written Ar. To calculate the relative formula mass of a compound, add together the realtive atomic masses of each element within the compound. 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