Back at it!

Well we are back at it! And bringing you today's topic...molarity!

Molarity is used to compare concentrations of solution. The concentration is the amount of solute dissolved in a certain volume of a solution. Molarity is the number of moles of solute in one litre of a solution, and we use the units moles/L.

Lets make sure you know the basics. A 10.00 moles/L solution is more concentrated than a 2.00 moles/L. Yes or no?
Hopefully you said yes. If you didn't, stop taking chemistry. Just kidding. But keep it in mind.
As always, we need a formula. Here's the one for molarity!

Molarity = moles of solute (mol)
                 ________________
                volume of solution (L)

or more simply if you need it:
M = mol
      -----
         L

Rearrange to solve for mol and L
mol = m x l and L = mol/M

Now onto some real questions.
Calculate the molar concentration of a solution that has 0.510 moles of NaOH in 1.400 L of solution.

Plug in the information given into the equation, and don't forget about the sig figs! We learned them for a reason people!

so M NaOH = 0.510 mol NaOH / 1.400 L
With the calculations, it comes out to M NaOH = 0.364286 M NaOH
But is that right? No!!!
The correct answer is 0.364 M NaOH.

There are three other types of questions we will cover:

1) calculating the number of grams
2) how many moles are contained in ___ L of ___ mole/L
3) What volume of ____ mole/L contains ___g of ___?

Lets start with calculating grams.
What is the number of grams of CaOH in 1.30 L of a 0.75 M Ca(OH)2 solution?
moles Ca(OH) 2 = 0.75 M Ca(OH) 2 x 1.30 L

We have to convert now, since it isn't in grams. So convert to grams.
Molar Mass = 40.1 + 17.0 x 2 = 74.1 g/mol
0.975 moles Ca(OH)2 x 74.1 g Ca(OH)2
                                     ------------------
                                     1 mole Ca(OH)2
So the answer is 72.2475 g Ca(OH)2
Don't forget the sig figs though! The answer is 72 g Ca (OH)2

Now, how many moles of Ca(OH) 2 are contained in 0.25 L of 0.80 mole/L Ca(OH)2?
0.8 x 0.25 = 0.20 mole Ca(OH)2

And thirdly, What volume of 0.50 mole/L Ca(OH)2 contains 0.10 g of Ca(OH)2?
volume required = moles
                            ------
                            concentration

1.7 x 10^-3 mole Ca(OH)2 / 0.50 mole/L Ca(OH)2
=0.0033 L

There you go! Now you can take some information and do four different things with it. Cool, right?
Well I know you have just been skimming this anyways waiting for the awesome videos, well they are coming at you now!

Basic Molarity Instructions
Need more help?
Molarity Rap (YOU MUST WATCH THIS!!)