Molecular Weight

Molecular weight, also known as molecular mass, is a measure of the total mass of all the atoms in a molecule. It is an important concept in chemistry and is used to calculate various properties of substances.

Calculating Molecular Weight

The molecular weight of a compound is calculated by adding up the atomic weights of all the atoms in the molecule. The atomic weight is the average mass of an atom of an element, taking into account the naturally occurring isotopes.

The molecular weight is usually expressed in atomic mass units (amu) or in grams per mole (g/mol).

Example Calculation

For example, the molecular weight of water (H₂O) can be calculated as follows:

Atomic weight of hydrogen (H) = 1.0079 amu

Atomic weight of oxygen (O) = 15.9994 amu

Molecular weight of water = 2(1.0079 amu) + 15.9994 amu = 18.0153 amu

Importance of Molecular Weight

Knowing the molecular weight of a substance is important because it allows us to calculate the amount of a substance needed for a reaction, the concentration of a solution, and various other properties such as density and molar volume.

Study Guide

Here are some key points to remember about molecular weight:

Molecular weight is the sum of the atomic weights of all the atoms in a molecule.
It is usually expressed in atomic mass units (amu) or in grams per mole (g/mol).
Molecular weight is used to calculate the amount of a substance needed for a reaction and other important properties.
To calculate the molecular weight of a compound, add up the atomic weights of all the atoms in the molecule.

Understanding molecular weight is essential in chemistry and is fundamental to many chemical calculations and analyses.

Hope this helps!

◂Physics Worksheets and Study Guides High School. Thermodynamics

The resources above cover the following skills:

PHYSICAL SCIENCE (NGSS)

Matter and Its Interactions

Students who demonstrate understanding can:

Develop a model to illustrate that the release or absorption of energy from a chemical reaction system depends upon the changes in total bond energy.

Energy

Students who demonstrate understanding can:

Create a computational model to calculate the change in the energy of one component in a system when the change in energy of the other component(s) and energy flows in and out of the system are known.

Plan and conduct an investigation to provide evidence that the transfer of thermal energy when two components of different temperature are combined within a closed system results in a more uniform energy distribution among the components in the system (second law of thermodynamics).