Atomic / Molecular Mass Calculation
6 Minutes
Atomic and molecular mass calculation for students explains relative atomic mass, molecular mass, and gram mass. An English science lesson.
- How Apollo 11 moon minerals were identified using atomic mass
- Calculating relative atomic mass and gram mass using carbon-12
- Finding relative and gram molecular mass of water and baking soda
What this lesson teaches
Relative atomic mass is a ratio that compares the mass of an individual atom to one-twelfth the mass of a carbon-12 atom. Because it is a ratio, relative atomic mass has no unit and is also called standard atomic weight.
Atomic mass and gram atomic mass
- The isotopic mass from the periodic table is rounded to a whole number for calculation, such as 1 for hydrogen and 16 for oxygen.
- Dividing this value by one-twelfth the mass of carbon-12 cancels out the mass units and gives the relative atomic mass.
- Because the units cancel during division, relative atomic mass is expressed as a pure number.
- Adding the unit of grams to this number gives the gram atomic mass, such as 1 gram for hydrogen and 16 grams for oxygen.
Finding relative molecular mass
A molecule is formed when more than one atom joins together. To find the relative molecular mass, the relative atomic values of every atom present in the molecule are added together and divided by one-twelfth the mass of carbon-12. For example, a water molecule contains two hydrogen atoms and one oxygen atom. Adding their values gives a relative molecular mass of 18, and adding grams gives a gram molecular mass of 18 grams.
Calculation for baking soda
Baking soda has the chemical formula NaHCO3, which contains sodium, hydrogen, carbon, and three oxygen atoms. The relative atomic mass is 23 for sodium, 1 for hydrogen, 12 for carbon, and 16 for each of the three oxygen atoms. Adding these values together gives 84, meaning the molecule is 84 times the mass of one-twelfth of carbon-12. Adding the gram unit gives a gram molecular mass of 84 grams for baking soda.
Identifying moon minerals
- Astronauts on the Apollo 11 mission brought rock and mineral samples from the moon back to Earth.
- Scientists used mass spectrometry to study the atomic masses of the elements inside those lunar samples.
- Calculating atomic masses allowed researchers to identify the unknown moon minerals as pyroxene and plagioclase.