Chemical Quantities

Note:

When doing calculations never clear your calculator.

Chemical Quantities

Why do chemists need to know the mass of an atom?

Mass of atomsThe mass of both protons and neutrons is approximately:

1.67 x 10-24 g

The mass of an electron is approximately:

9.076 x 10-28 g

These masses are difficult to work with!So chemists developed a method of measuring the mass of an atom relative to the mass of a specific atomic standard.

Mass of atomsThe standard used is the carbon-12 isotope

Chemists assigned the carbon-12 atom a mass of exactly 12 atomic mass units, 12 amu.

= 12 amu- -

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Relative Atomic Mass, ArThe relative atomic mass, Ar , is the average mass of an atom of an element compared to a carbon-12 atom.

It is a ratio so it has no units.

For example:

Ar (H) = 1 meaning a H atom has a mass 1/12 of C-12.

Ar (Na) = 23 meaning a Na atom has a mass 23/12 of C-12.

Relative Molecular Mass, Mr

The relative molecular mass, Mr , is the average mass of a molecule compared to a carbon-12 atom.

It is the sum of all the relative atomic masses, Ar.

It is a RATIO so it has no units.

Mr(H2O) = 2 x Ar(H) + Ar(O)

= (2 x 1) + 16= 18