UNIT # 7 Introduction Objectives Problems 1 | 2 | 3 | 4 | 5

CALCULATIONS BASED ON FORMULAE

7.3 - Atomic Mass Revisited

In section 3.5 we learned about atomic mass and the unit of amu was introduced. Take some time and review before moving on. It is important that you fully understand the concept of atomic mass unit.

Section 4.5
Isotopes ..p85

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Go on ... come back to this page when you're ready ... we're in section 7.3 ... I'll wait.

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In section 7.2 we introduced the idea of the mole and told you why we need to scale up to this unit of counting. Now that you have some ideas of just how big a mole of substance (like water) is, we use this scaled up unit of counting in order to refer to a mass of a water molecules that we can physically see. The real neat thing about Avogadro's number is that:

We can equate the numerical quantity of the mass of a single ATOM (in amu) to the mass of a mole of the same atoms (in grams).

For example:

1 carbon atom

has a mass of 12.011 amu.

 1 mole, or 6.022x 1023, of carbon atoms has a mass of 12.011 grams.

Points to consider:

• The atomic mass of one atom has units of atomic mass unit, amu.
• The atomic mass of one mole of atoms has units of grams, g.

Here is another example:

1 sodium atom

has a mass of 22.990 amu.

 1 mole, or 6.022x 1023, of sodium atoms has a mass of 22.990 grams.

Again,

• The atomic mass of one atom has units of atomic mass unit, amu.
• The atomic mass of one mole of atoms has units of grams, g.

The units of atomic mass can be:

Section 8.1
Calculation of Mole of Units. Avogadro's Number (N) ..p189

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