Unit 1.2: Formulae, equations and moles
 |
| Mole Related Avagadro's Constant |
Learning bit:-
Avogadros constant is the number of carbon atoms in exactly 12g of the carbon-12
isotope. It is equal to 6.02x 10^23 mol^-1.
One
mole of a substance is the amount of that substance
that contains 6.02x 10^23 particles of that substance. This means that one mole
of a substance is its relative atomic or molecular mass expressed in grams.
The molar mass of a substance is the mass
(in grams) of one mole.
Amount
of substance is the number of moles of that
substance.
The empirical formula is the simplest whole number ratio of
the elements in the compound.
Calculation of empirical formulae from percentage data
Easy way of calculating it: -
Element
|
% of element
|
%/Ar
|
Divide by lowest
|
Carbon
|
48.7
|
48.7/12 = 4.1
|
4.1/2.7=1.5
|
Hydrogen
|
8.1
|
8.1/1 = 8.1
|
8.1 / 2.7 = 3
|
Oxygen
|
43.2
|
43.2 / 16 = 2.7
|
2.7 / 2.7 = 1
|
The last column gives the empirical
formula, but if any value in this column comes to a non-integer value, multiply
it by 2 or 4 etc
So here, the empirical formula is C3H6O2.
Equations
These must balance. The number of atoms of
an element on one side of the equation must be = to the other side
Ionic equations
3 rules: -
- Write the ions separately
for solutions of ionic compounds
- Write full ‘molecular’ formulae for solids and all covalent substances
- Spectator ions must
be cancelled and so do not appear in the final
equation.
***they must also balance for charge***
Moles
There are 3 ways of calculating the amount
of substance (in moles):
- For pure substance (X)
Amount of X (in moles) = mass of X (in grams)/its molar mass
- For solutions:
The amount of solute = concentration (in mol dm^-3) x volume (in dm^3)
- For gases:
Amount of gas (in moles) = volume (in dm^3)/molar volume
***the molar volume of a gas is 24 dm^3 mol^-1,
measured @ standard conditions***
Calculation of number of
particles
The no of
particles can be calculated from the number of moles.
·
The number of molecules = moles * avogadros
constant
·
The number of ions = moles * avogadros constant *
number of ions in the formula
Other stuff
Concentration of solutions: -
This
is either: amount of solute (in moles)/
volume of solution in dm^-3
units:
mol dm^-3
or:
mass
of solute (in grams)/ volume of solution in dm^-3
units:
g dm^-3
oh, and DON’T round up numbers during the
calculation!!
The above is all what you really need to
know. Its just about applying to different situations. Such as titrations etc.
Try writing out what you have before attempting the question.
The only way to get good at these questions
are by practice!!! They are easy marks in the exam! J
0 comments:
Post a Comment