Molecular Mass Quiz

Molecular Mass

• Molecular mass is the sum of the atomic masses of all atoms in a molecule.
• It is expressed in atomic mass units (amu)
• To calculate molecular mass, find the atomic mass of each element in the molecule from the periodic table.
• Multiply the atomic mass by the number of atoms of that element present in the molecule.
• Sum the products to find the total molecular mass.

Atomic Mass

• Atomic mass is the weighted average mass of all isotopes of an element.
• It is expressed in atomic mass units (amu).
• The weight is based on the relative abundance of each isotope in nature.
• Isotopes are atoms of the same element with different numbers of neutrons but the same number of protons.

Molar Mass

• Molar mass is the mass of one mole of a substance.
• It is expressed in grams per mole (g/mol).
• One mole of any substance contains Avogadro's number of particles (approximately 6.022 × 10²³).
• To calculate molar mass, essentially use the same method as for molecular mass.
• The units essentially translate to the mass of 6.022 × 10²³ atoms or molecules of the species.

Relationship between Molecular Mass and Molar Mass

• Molecular mass and molar mass have the same numerical value.
• The difference is in the units
• Molecular mass is expressed in atomic mass units (amu) whereas molar mass is expressed in grams per mole (g/mol).

Calculating Molecular Formulas from Experimental Data

• The actual molecular formula represents the exact number of atoms of each element in a molecule.
• Empirical formulas give the simplest whole-number ratio of atoms present in a compound.
• Experimental data, such as combustion analysis, can provide the percent composition by mass of elements in a compound.
• These data can be used to derive the empirical formula, a simplified formula, and then further advanced to determine the molecular formula.

Steps to Determine Molecular Formula:

• 1. Assume a 100 g sample and calculate the moles of each element using the percentage composition and their respective molar masses.
• 2. Determine the simplest whole number ratio by dividing each element's mole value by the smallest mole value in the ratio to obtain moles ratio.
• 3. Convert the moles ratio to subscripts of each element in the empirical formula.
• 4. Calculate the empirical formula mass (molar mass of empirical formula).
• 5. Find the molecular mass from experimental data (or from other sources).
• 6. Calculate the ratio of the molecular mass to the empirical formula mass.
• 7. Multiply the subscripts of each element in the empirical formula by the ratio calculated in the previous step to determine the molecular formula.

Example :

• Suppose a compound contains 40% carbon, 6.7% hydrogen and 53.3% oxygen.
• Calculate the empirical formula.
• Then assume the molecular mass is found to be 60 g/mol and deduce the molecular formula.