Molarity
Molarity finds the concentration of a solution. Concentration is how many particles of the solute are there in a solvent.
Here's the formula:
Molarity (M) = Moles of Solvent
Litres of Solution (L or dm^3)
When working out, use mol/dm^3, and use M when writing the final answer.
The formula is pretty simple and you do not have to overcomplicate it. The higher the number, the more concentrated the solution is. It's that simple!
How does it work?
Let's say you're working as an intern for an evil scientist and he tells you he needs a solution of 2M CHCl3 (which is chloroform by the way). What you would do is (ratio may differ) 1) Obtain 2 moles of Chloroform 2) Put it in a measuring cylinder 3) Then fill it with water until the solution reaches 1 L.
The difference here is we don't really care about the amount of solvent, all we care is the amount of solution that comes out. Word of warning
Notice the denominator says "Litres of Solution"! Remember, solution is everything, which includes the solvent and the solute, added together. Let's try a practice question and you'll see what I mean!
Practice
Calculate the molarity of a solution prepared by dissolving 23.7 grams of KMnO4 into enough water to make 750 mL of solution.
1) Look back at the formula. We need moles for this, so let's convert everything into moles. (Forgot me?)
First, find the RFM of KMnO4.
Molar mass of K = 39.1 g Molar mass of Mn = 54.9 g Molar mass of O = 16.0 g Molar mass of KMnO4 = 39.1 g + 54.9 g + (16.0 g * 4) Molar mass of KMnO4 = 158.0 g
Then we divide the given mass by our RFM to find the amount of moles:
23.7/158 = 0.15 mol
2) Now we need to find the litre of solution.
Now this was what I was rambling about. Most students will get stuck or tricked by water. Water is the solvent, not the solution! Now we know KMnO4 dissolved into water gives 750mL of solution. So do not care about what's "enough" water. Remember, it's the solution that counts, and you know in this case the amount of solution is 750 mL. So let's convert that to L.
750 mL = (750/1000) L = 0.75 L of solution
3) What's the molarity?
Simply plug everything into the formula:
0.15/0.75 = 0.2 M
So now we know the molarity of this solution is 0.2M.
The general steps:
1) Convert grams to moles, then find the number of moles of solute
2) Convert mL or any other unit to L or dm^3 - remember, solution not solvent!
3) Divide moles by litres
Now let's move on to Molality!
