Relative Atomic, Isotopic, Molecular, and Formula Mass

Question-and-answer flashcards covering definitions and example calculations for relative atomic, isotopic, molecular, and formula masses.

What is relative atomic mass (A_r)?

The average mass of an atom of an element on a scale where a carbon-12 atom has a mass of exactly 12.

What is relative isotopic mass?

The mass of an atom of a specific isotope on a scale where a carbon-12 atom is exactly 12.

How do you calculate relative atomic mass from isotopic masses?

Multiply each isotopic mass by its percentage abundance, add the results, then divide by the total percentage (usually 100).

Using 75 % 35Cl and 25 % 37Cl, what is chlorine’s relative atomic mass?

(35 × 75 + 37 × 25) / 100 = 35.5

What is relative molecular mass (M_r)?

The average mass of a molecule on a scale where a carbon-12 atom has a mass of exactly 12; equal to the sum of the relative atomic masses of all atoms in the molecule.

How do you calculate the relative molecular mass of C₂H₆O?

(2 × 12.0) + (6 × 1.0) + (1 × 16.0) = 46.0

What is the relative molecular mass of C₄H₁₀?

(4 × 12.0) + (10 × 1.0) = 58.0

What is relative formula mass?

The average mass of a formula unit on a scale where a carbon-12 atom is exactly 12; used for ionic and giant covalent compounds.

How is relative formula mass calculated?

Add up the relative atomic masses of all ions (or atoms) in the formula unit.

What is the relative formula mass of CaF₂?

40.1+ 2 × 19.0 = 78.1

What is the relative formula mass of CaCO₃?

40.1 + 12.0 + 3 × 16.0 = 100.1

What is the difference between relative molecular mass and relative formula mass?

Relative molecular mass is used for molecules, whereas relative formula mass is used for substances that are not made of discrete molecules, such as ionic or giant covalent compounds.