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

CALCULATIONS BASED ON FORMULAE

7.4 - Calculation of Molar Mass

In section 7.1 we learned how to calculate the mass of a formula unit or a molecule. Now we will learn how to calculate the mass of 1 mole of substance. In section 7.3, we saw that the real neat thing about Avogadro's number is that:

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

or, when we are talking about molecules,

We can equate the numerical quantity of mass of a single MOLECULE to the mass of a mole of the same molecules (in grams).

For example:

1 water molecule, H2O

has a mass of 18.015 amu.

 2 H atoms: 2 x 1.008 amu = 2.016 amu 1 O atom: 1 x 15.999 amu = 15.999 amu Total mass: 18.015 amu

 2 moles of H atoms: 2 x 1.008 grams = 2.016 grams 1 mole of O atom: 1 x 15.999 grams = 15.999 grams Total mass: 18.015 grams
1 mole, or 6.022x 1023, of water molecules has a mass of 18.015 grams.

This is the molar mass of water.

The units of molar mass:

Let's take a look at the determination of the molar mass of sodium carbonate, Na2CO3.

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