 # Mass calculations for SQA National 5 Chemistry 1. Conservation of mass
2. Relative formula mass
3. Percentage mass

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. # 1

What is the percentage mass of oxygen in CO₂?

(16 x 2) / 44 = 72.73% # 2

Work out the missing element X in compound Ca(XO₃)₂. It has a molecular mass of 164 g/mol.

40 + 2(X + 3 × 16) = 164
2X = 28
X = 14
X is N (nitrogen)

nitrogen # 3

What does the law of conservation of mass state?

No atoms are lost or made in a chemical reaction. # 4

What is the relative formula mass of C₆H₁₂O₆?

(12*6) + 12 + (16*6) = 180 # 5

What is the abbreviation for relative formula mass?

Ar End of page