Molecular Weight

The molecular weight of a substance is the sum of the atomic weights of all the atoms in a molecule. It is an important concept in chemistry as it helps in determining the amount of a substance needed for a given reaction or in calculating the concentration of a solution. The molecular weight is expressed in atomic mass units (amu) or in grams per mole (g/mol).

Calculating Molecular Weight

To calculate the molecular weight of a compound, you need to know the chemical formula of the compound. Then, you can use the following steps:

1. Identify the elements present in the compound and their respective atomic weights.
2. Count the number of each type of atom in the compound.
3. Multiply the atomic weight of each element by the number of atoms of that element in the compound.
4. Add the products from step 3 to get the molecular weight of the compound.

Example Calculation

Let's calculate the molecular weight of glucose (C6H12O6).

• Atomic weight of carbon (C) = 12.01 amu
• Atomic weight of hydrogen (H) = 1.008 amu
• Atomic weight of oxygen (O) = 16.00 amu

Now, using the formula C6H12O6:

Molecular weight = (6 x 12.01) + (12 x 1.008) + (6 x 16.00) = 180.156 amu

Significance of Molecular Weight

The molecular weight of a substance is important in various aspects of chemistry, including:

• Stoichiometry: It is used to balance chemical equations and calculate the amount of reactants needed or products formed in a reaction.
• Molar mass: It is used to convert between the mass of a substance and the number of moles present.
• Solute concentration: It is used to calculate the concentration of a solute in a solution.

Study Guide

When studying molecular weight, it's important to understand the following key points:

1. The concept of atomic weight and its significance in calculating the molecular weight of a compound.
2. How to calculate the molecular weight of a compound by following the step-by-step process.
3. The practical applications of molecular weight in chemistry, such as in stoichiometry, molar mass calculations, and concentration calculations.

Understanding molecular weight is essential for mastering various topics in chemistry, so be sure to practice calculating molecular weights for different compounds and familiarize yourself with its applications in different contexts.