Chemistry Notes

1. The Mole

Don’t forget that individual atoms are far too small to weigh!

ü  Chemist’s ‘dozen’ is the mole

ü  One ‘dozen’ means 12 bananas.

ü  One ‘moles’ means 6.02 x 10²³ of any particles,called (Avogadro’s Number )

ü  A mole = 22.4 litres = molecular mass of gas or liquid.

ü  A mole = relative atomic mass of an element.

1 mole of zinc atoms (ANY particles!) would have a mass of 65.4g (This is the mass number printed on your periodic table) and so on.

1 mole of zinc atoms (I repeat ANY particles!) contains 6.02 x 10²³ atoms.

1 mole of methane molecules (I repeat again ANY particles!) contains 6.02 x 10²³ molecules.

Notice the following:

Average weight: If 75% of all chlorine atoms are Cl-35 and 25% are Cl-37, then the weighted average is:-

(0.75 × 35) + (0.25 × 37) = 35.5.

The mole is a number that answers the question, "How many?", in the same way a meter answers the question, "How far?", or a kilogram answers the question, "How much mass?"

For example, sodium chloride has the formula NaCl. When 6.02 x 10²³ sodium ions and 6.02 x 10²³ chloride ions combine together we have 1 mole of sodium chloride. This means that:

a.      One mole of NaCl contains one mole of sodium ions and one mole of chloride ions.

b.      One mole of Na is the same amount as 22.9898 grams.

c.      One mole of Cl is the same amount as 35.453 grams.

d.      One mole of NaCl is the same amount as 22.9898 + 35.453 grams, or 58.4418 grams. It follows that:

§  1/2 mole of NaCl is 29.2209 grams.

§  2 mole of NaCl is 104.8836 grams.

§  10 mole of NaCl is 584.418 grams.

Note: The mass of an electron is negligible compared to the rest of the atom. Compare this to another ionic compound, K₂O. This compound contains two moles of potassium ions, K⁺, and one mole of oxide ions, O^2-, in one mole of potassium oxide.

*The formula of ionic compound does not represent its molecule. It simply represents a unit of lattice.

*One mole NaCl = 6.022 X 10²³ units of NaCl  i.e. each units of NaCl = 6.022 X 10²³ particles of Na⁺ and 6.022 X 10²³ particles of Cl^- .

At last for you!

How can I Convert mole into weight which I need for my experiment?

Use this relation:

     Mole = Given weight / Molecular weight

 

Do you know? That – We retain information at these rates:

ü  10 percent of what we read.

ü  20 percent of what we hear.

ü  30 percent of what we see.

ü  50 percent of what we see and hear.

ü  70 percent of what we see and discuss.

ü  90 percent of what we do.

Another way to think about how we retain information is this edge:

Tell me and I will probably forget,

Show me and I might remember,

Involve me and I will learn.

Ha ha ha ! I am going to involve you to solve my problems!

Problems related to moles:

A – Theory:

Short answers type questions:

ü  How is the mass of an element related to the number of atoms present in it?

ü  Which does have more atoms 1.0 g of hydrogen or 1.0 g of oxygen?

ü  What is Avogadro’s number and what is its significance?

ü  How is the mole concept useful in calculations based on chemical equations?

Is it true or false?

ü  One mole of K₂SO₄ contains 6.02 X 10²³ potassium ions.

ü  One gram mole of a monoatomic gas occupies 22.4 L at S.T.P.

ü  A chemical equation tells the actual number of moles of reactants and products.

ü  The number of atoms present in 1 g of hydrogen is the same as that in 1 g of oxygen.

ü  The ratio of atoms can vary in differently prepared samples of a compound.

ü  How many moles of chromium ions and oxide ions are present in one mole of Chromium (III) oxide, Cr₂O₃?

Fill in the blanks:

ü  One gram mole, one gram molecules, and one gram molecular mass have the …………..meaning.

ü  A mole is defined as the amount of the substance that contains as many specified elementary particles as the number of atoms in ……………..g of C¹².

ü  Total numbers of atoms in 90 g of water are…………

ü  The weight of 1 X 10²² molecules of CuSO₄.5H₂O is………….

ü  The volume occupied by the 64 g of SO₂ at S.T.P. is………………