Relative Atomic Mass and Relative Molecular Mass

Vocabulary flashcards covering the definitions, formulas, standard reference reasons, and calculations for Relative Atomic Mass (RAM), Relative Molecular Mass (RMM), and Relative Formula Mass (RFM).

Relative Atomic Mass (RAM)

The average mass of an atom of the element compared to $\frac{1}{12}$ of the mass of one carbon-12 atom.

Formula for Relative Atomic Mass of an Element

$$\frac{\text{Average mass of one atom of the element}}{\frac{1}{12} \times \text{Mass of one carbon-12 atom}}$$

Reasons Carbon-12 is Used as the Standard Reference

1. Carbon-12 is a solid at room temperature and thus can be handled easily. 2. Carbon-12 combines easily with other elements. 3. Carbon-12 is found in most substances.

Relative Molecular Mass (RMM)

The average mass of the molecule compared to $\frac{1}{12}$ of the mass of one carbon-12 atom.

Formula for Relative Molecular Mass of a Substance

$$\frac{\text{Average mass of one molecule}}{\frac{1}{12} \times \text{Mass of one C-12 atom}}$$

Calculation of Relative Molecular Mass (RMM)

Calculated by adding up the relative atomic masses of all the atoms that form the molecule.

RMM of Oxygen Gas, $O_2$

$$2(\text{RAM of O}) = 2(16) = 32$$

RMM of Water, $H_2O$

$$2(\text{RAM of H}) + \text{RAM of O} = 2(1) + 16 = 18$$

Relative Formula Mass (RFM)

The relative mass of an ionic substance.

Calculation of Relative Formula Mass (RFM)

Calculated by summing up the relative atomic masses of all the atoms shown in the formula of the ionic substance.

RFM for Zink Chloride, $ZnCl_2$

$$\text{RAM of Zn} + 2(\text{RAM of Cl}) = 65 + 2(35.5) = 65 + 71 = 136$$