Molecular Mass Quiz

Study Notes

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.
1. 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.
1. Convert the moles ratio to subscripts of each element in the empirical formula.
1. Calculate the empirical formula mass (molar mass of empirical formula).
1. Find the molecular mass from experimental data (or from other sources).
1. Calculate the ratio of the molecular mass to the empirical formula mass.
1. 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.

Description

Test your understanding of molecular mass, atomic mass, and molar mass with this quiz. You'll need to calculate and interpret these important concepts in chemistry, including how to use the periodic table effectively. Perfect for students looking to solidify their knowledge in the subject!