Chris' Chemistry Calculations
It's like magic, but not.
Most notes either taken from my own
notes, the CGP revision guide, I didn't
use the textbook much because it sucks.
First off, the easy stuff, moles.
Before we dive right in, why not grab a pen and paper?
Copy some notes take part in my fab examples.
Or don't, whatever.
The Main equation you need to learn is this:
Mass
rfm/ram
An example question using this equation could simply be:
You have stolen 120g of iron off your mother. You swine.
How many moles is this?
To start you'll need to substitute in the reletative atomic mass and mass (g) of iron into that ever so simple equation.
Mass
rfm/ram
120g
55.8
=
2.15mol (to 3sf)
Yes, its that simple.
However if you're dealing with those darn solutions, this equation is your man:
volume
1000
x concentration
Questions using this equation are also fairly simple. Here's a couple to tingle your tinglers.
You've found yourself a lovely solution, it tastes good and theres 300cm,
it's got a smooth concentration of 3mol dm
How many moles?
3
-3
x concentration
volume
1000
300
1000
x 3
0.9mol. Lovely.
An old man tragically spills some solution on your thigh
without getting distracted by his sensual rubbing as he wipes
the stain off, you try to work out the concentration.
Thankfully this friendly guy has gave you some vital info;
the solution's volume is 250cm and there's a cool 1.5 moles.
3
Turns out you'll have to rearrange the equation this time seen as you're trying to work out the concentration.
Moles x 1000
Volume
= Concentration
1.5 x 1000
250
= Concentration
6mol dm
-3
While we're still talking moles, lets bring gases into the 'equation'
Main thing to remember when working with gases, they all use 24dm per mole.
3
So to work out the number of moles in a gas
use the first normal equation but replace rfm with 24dm.
3
Volume/24dm
3
Emperical and Molecular formula
Emperical is the simplest ratio
of atoms in a compound e.g CO
2
Molecular is the actual number of
atoms in a compound e.g. C O
2
4
Questions related to Emperical formula can be worth a lot of
marks, which is nice, because it can be fairly simple with a good technique.
e.g: a) You have a compound with;
56.5% K, 8.7% C and 34.8% oxygen
What's it empirical formula?
The first thing I'd do would to write out a table with the following headings;
Element % RAM Moles Simplest Ratio
Then fill it with the appropriate information,
Element % RAM Moles Simplest Ratio
K 56.5 39.1 1.445 2
C 8.7 12 0.725 1
O 34.8 16 2.175 3
Remember, moles = Mass/RAM
In this case, use the percentage
in place for mass.
To work this out divide all the moles by the smallest amount (0.725 here).
Emperical formula:
K CO
2
3
b) The relative formula mass of this compound is 414.6 - What's the molecular formula?
2
K CO
3
This is an easy way to pick up marks, and even easier if everything went okay in the first part.
To start you'll need to work out the relative formula mass of our empirical formula.
138.2
And we're looking for 414.6
Divide that by our previous empirical mass;
414.6/138.2 = 3
Easy enough, all we need to do now is
multiply all our elements in our empirical
formula by 3 to get our molecular.
K C O
6
3
9
With the second part, this can be worth a fair bit of marks. Empirical formula
will appear in your exam one way or another, either a 1 Mark multiple choice
or a 4 mark structured question, it's worth knowing.
Ionic Equations
e.g. Write the ionic equation for Iron and Copper Sulfate
First step is to write the normal equation;
Fe + CuSO
FeSO + Cu
(s)
4 (aq)
4 (aq)
(s)
Next, ions need to be involved.
To work out each charge, remember just to use your periodic table;
Group 1 elements have +1 Charge
Group 2 elements have +2 Charge
Group 7 elements
have -1 Charge
etc.
It'd be best to learn some compound ions too;
NO
3
-
Nitrate
CO
3
2-
Carbonate
SO
4
2-
Sulfate
NH
4
+
Ammonium
Back to our example...
You now have to write in the charges;
Fe + Cu + SO
2+
4
2-
Fe + Cu + SO
2+
2-
4
The same on both sides, these are the
Spectator Ions
Spectator Ions take no part in the reaction - same on both sides, so re-write the equation without them.
Fe + Cu
2+
Fe + Cu
2+
Ionic Equations,
Done.
Next up,
Reacting Masses
Example:
How many tonnes of limestone have to be heated to produce
100 tons of calcium oxide? Don't worry, I couldn't care less either.
Limestone = CaCO
3
CaCO
CaO + CO
2
3
You need 100tonnes of this stuff
lets put that into moles;
Mass/RFM
100/56
1.79
The mole to mole ratio, is a simple 1:1 thus;
1.79 = Mass(t)/100
rfm
= 179tonnes
That's your answer, in mass NOT moles.
Moles
Atom Economy
Not much to learn here, one easy equation;
mass of desired product
mass of all reactants
Atom economy details the efficency of the reaction.
This is better explained in an example:
Calculate the atom economy of this reaction;
2Al(OH)
Al O + 3H O
2
3
3
2
mass of all reactants
mass of desired product
_________________________________________
=
(27 x 2) + (16 x 3)
_________________________________________
((27 x 2) + (16 x 3)) + 3 x ((1 x 2) + 16)
x 100
Atom Economy is a percentage so
you'll need to multiply by 100.
=
65.4%
That's everything!
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theTechLife.co.uk
Youtube ReadySaltedChirss, dailybooth @ReadySaltedChris, and twitter @ChrisFTW
Thanks and Good Luck!
- Chris