Mass Number: Found in the top left corner of an element's nuclear symbol; it's the sum of protons and neutrons.
Relative Atomic Mass (Ar): The average mass of all the isotopes of an element, denoted as 'Ar'.
Example: Chlorine has isotopes with mass numbers 35 and 37, but an Ar of 35.5 due to the average considering isotope abundance.
Relative Formula Mass (Mr): The sum of the relative atomic masses of all atoms in a compound's molecular formula, denoted as 'Mr'.
Calculating Relative Formula Mass
Magnesium Chloride (MgCl2):
Magnesium Ar: 24
Chlorine Ar: 35.5
Calculation: 24 + (35.5 * 2) = 95
Sulfuric Acid (H2SO4):
Hydrogen Ar: 1
Sulfur Ar: 32
Oxygen Ar: 16
Calculation: (2 * 1) + (1 * 32) + (4 * 16) = 98
Calculating Percentage Mass of an Element in a Compound
Formula:
Percentage Mass = (Ar of element * number of atoms of element / Mr of compound) * 100
Example: Percentage Mass of Sulfur in Sulfuric Acid
Ar of Sulfur: 32
Mr of H2SO4: 98
Calculation: (32 * 1 / 98) * 100 = 32.7%
Percentage Mass Calculation Example in Calcium Hydroxide
Calcium Hydroxide (Ca(OH)2):
Calcium Ar: 40
Oxygen Ar: 16
Hydrogen Ar: 1
For Oxygen: (16 * 2)
Mr Calculation: 40 + (2 * 16) + (2 * 1) = 74
Percentage Mass of Oxygen: (32 / 74) * 100 = 43.2%
Conclusion
Understanding how to find the relative formula mass and calculating percentage mass of an element in a compound are essential skills in chemistry. Use the periodic table to find Ar values and apply them to the formulas provided.