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Calculadora Pesos Moleculares

Reviewed by Calculator Editorial Team

The molecular weight (also known as molar mass) of a compound is the sum of the atomic weights of all the atoms in its molecular formula. This calculator helps you determine the molecular weight of any chemical compound by entering its molecular formula.

What is Molecular Weight?

Molecular weight, often referred to as molar mass, is a fundamental concept in chemistry. It represents the mass of one mole of a substance and is expressed in grams per mole (g/mol). The molecular weight of a compound is crucial for various chemical calculations, including stoichiometry, solution preparation, and gas law problems.

The molecular weight is calculated by summing the atomic weights of all the atoms in the molecular formula. Atomic weights are based on the relative atomic masses of the elements, which are derived from the periodic table.

How to Calculate Molecular Weight

Calculating the molecular weight of a compound involves the following steps:

  1. Write down the molecular formula of the compound.
  2. Identify each element in the formula and determine its atomic weight from the periodic table.
  3. Count the number of atoms of each element in the formula.
  4. Multiply the atomic weight of each element by the number of atoms of that element.
  5. Sum the results to obtain the molecular weight.

Remember that molecular weight is different from molecular mass. Molecular mass refers to the mass of a single molecule, while molecular weight is the mass of a mole of molecules.

Molecular Weight = Σ (Atomic Weight × Number of Atoms)

Examples of Molecular Weight Calculations

Let's look at a few examples to understand how to calculate molecular weight.

Example 1: Water (H₂O)

To calculate the molecular weight of water:

  1. Identify the elements: Hydrogen (H) and Oxygen (O).
  2. Determine the atomic weights: H = 1.008 g/mol, O = 16.00 g/mol.
  3. Count the atoms: 2 hydrogen atoms and 1 oxygen atom.
  4. Calculate: (2 × 1.008) + (1 × 16.00) = 2.016 + 16.00 = 18.016 g/mol.

The molecular weight of water is approximately 18.02 g/mol.

Example 2: Methane (CH₄)

To calculate the molecular weight of methane:

  1. Identify the elements: Carbon (C) and Hydrogen (H).
  2. Determine the atomic weights: C = 12.01 g/mol, H = 1.008 g/mol.
  3. Count the atoms: 1 carbon atom and 4 hydrogen atoms.
  4. Calculate: (1 × 12.01) + (4 × 1.008) = 12.01 + 4.032 = 16.042 g/mol.

The molecular weight of methane is approximately 16.04 g/mol.

Example 3: Sodium Chloride (NaCl)

To calculate the molecular weight of sodium chloride:

  1. Identify the elements: Sodium (Na) and Chlorine (Cl).
  2. Determine the atomic weights: Na = 22.99 g/mol, Cl = 35.45 g/mol.
  3. Count the atoms: 1 sodium atom and 1 chlorine atom.
  4. Calculate: (1 × 22.99) + (1 × 35.45) = 22.99 + 35.45 = 58.44 g/mol.

The molecular weight of sodium chloride is approximately 58.44 g/mol.

Frequently Asked Questions

What is the difference between molecular weight and molecular mass?

Molecular weight refers to the mass of one mole of a substance, while molecular mass refers to the mass of a single molecule. Molecular weight is expressed in grams per mole (g/mol), whereas molecular mass is expressed in atomic mass units (u).

How do I find the atomic weights of elements?

You can find the atomic weights of elements in the periodic table. Each element has a specific atomic weight value, which is typically listed below the element's symbol.

Can I use this calculator for any chemical compound?

Yes, this calculator can be used for any chemical compound as long as you know its molecular formula. Simply enter the formula, and the calculator will compute the molecular weight for you.

Is molecular weight the same as molar mass?

Yes, molecular weight and molar mass are essentially the same concept. They both refer to the mass of one mole of a substance and are expressed in grams per mole (g/mol).