But first, let's understand a hypothesis of a great Italian scientist named Avogadro.
He proposed that equal volumes of different gases at the same temperature and pressure will have the same number of particles.
Now, let's define Atomic Mass.
This is the mass of 1 atom of the element in the units amu. (atomic mass units)
For example, if you look at the atomic mass of an element on the periodic table, it will give you how much 1 atom of that element weighs..
So, let's take Carbon. Carbon's atomic mass is 12.0 amu. This means that every carbon atom weighs 12 atomic mass units.
Now formula mass. This means you add up all the masses of the atoms in the formula of an ionic compound.
So.... let's say we have NaCl.
Na is sodium, and the atomic mass is 23.0.
Cl is chlorine, and the atomic mass is 35.5.
So by simple addition: 23.0 + 35.5 = 58.5 amu. This means that NaCl weighs 58.5 atomic mass units.
Molecular Mass: All the atoms of a formula in a covalent compound are added in amu.
So: Carbon Dioxide is
C + O2
12.0 + 16.0X2
CO2 = 44.0 amu
Molar Mass is the atomic, molecular, or formula mass of any pure substance, and the unit is in grams per mole.
For example, 1 mole of oxygen is just its atomic mass (16.0 amu, but in a different unit, g/mol.)
So 1 mole of oxygen = 16.0 g/mol
1 mole of carbon = 12.0 g/mol
They both have the same number of particles. Hence, the molar atomic mass of an element is the mass of 1 mole of that element.
It's just a unit that helps us and scientists count atoms and molecules, without actually counting every single atom.
And what Avogadro found, was that the number of particles in 1 mole of any amount of substance is always 6.022 X 10^23 particles per mole.
Now here's a video if you're still somewhat confused.
And here's a website to summarize the different types of masses to calculate.
http://www.mpcfaculty.net/mark_bishop/molar_mass_conversion_factors_help.htm
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