Question

a. Define formula mass. b. In what unit is formula mass expressed?

Short answer

Formula mass is the sum of the average atomic masses of all atoms in a formula, expressed in amu.

Step-by-step solution

Understand Formula Mass

Formula mass is the sum of the average atomic masses of all atoms represented in the chemical formula of a substance. It reflects the weight of a single molecule or formula unit of a compound.

Identify Units of Formula Mass

The unit of formula mass is expressed in atomic mass units (amu). This unit measures the mass of atoms and molecules and is based on the mass of one-twelfth the mass of a carbon-12 atom.

Key concepts

Understanding Average Atomic Masses

The concept of average atomic masses is fundamental in calculating the formula mass of a compound. Each element can have different isotopes, which are atoms of the same element with different numbers of neutrons. These isotopes contribute to the average atomic mass of an element, depending on their relative abundance in nature.
The average atomic mass of an element is calculated by multiplying the mass of each isotope by its natural abundance expressed as a decimal, and then summing these values. This average value is what you typically see on the periodic table.
For example, chlorine has two main isotopes: chlorine-35 and chlorine-37. If 75% of chlorine atoms are chlorine-35 and 25% are chlorine-37, the average atomic mass of chlorine would be calculated as
\[(0.75 \times 35 \text{ amu}) + (0.25 \times 37 \text{ amu}) = 35.5 \text{ amu}\].

Chemical Formula

A chemical formula represents the types and numbers of atoms in a molecule or compound. It's a shorthand way of showing the composition.
There are different types of chemical formulas:

• Empirical formulas show the simplest whole-number ratio of atoms in a compound.
• Molecular formulas indicate the actual number of atoms in a molecule.
• Structural formulas depict the bonding arrangement between atoms.

For example, the empirical formula for hydrogen peroxide is HO, while its molecular formula is \(\text{H}_2\text{O}_2\). These formulas are essential for calculating formula mass, as they tell you how many atoms of each element are in a single molecule of the substance.

Atomic Mass Units (amu)

Atomic Mass Units (amu) is the standard unit of measurement used to express the mass of atoms and molecules.
One atomic mass unit is defined as one twelfth the mass of a carbon-12 atom. This standard is used because carbon-12 is the most common isotope of carbon and provides a convenient reference point.
Using amu simplifies the way we measure atomic and molecular masses in chemistry. Instead of dealing with extremely small numbers, we use a scale that makes these measurements more manageable. For example, instead of saying the mass of a hydrogen atom is 1.67 \(\times 10^{-24}\) grams, we say it is approximately 1 amu.

The Carbon-12 Atom

The carbon-12 atom is a specific isotope of carbon that has 6 protons, 6 neutrons, and 6 electrons.
It serves a significant role in the definition of atomic mass units (amu). The mass of one carbon-12 atom is exactly 12 amu, making it a natural standard for measuring the masses of other atoms.
This standardization means that the atomic mass unit is based on 1/12th of the mass of a carbon-12 atom, creating consistency and simplicity in the way atomic masses are calculated and compared.
When you see atomic masses on the periodic table, they are often described in relation to carbon-12. Thus, understanding carbon-12 is key to grasping how atomic masses are determined and used.