Welcome to molarity, in which we will explore what concentration is, how molarity is expressed
and how molarity is used in calculations, along with some definitions.
I guess the animals are getting out of hand today.
So, what is concentration?
Well, first we need some water, but let's get a second glass to dissolve something into,
and define concentration as how much substance is dissolved in a given solvent.
Dissolving a substance in water creates a solution.
If we dissolve even more substance, than we get a solution with a higher concentration,
and by comparison, the other solution has a lower concentration.
Before we go further, let's define some of the components of our illustration.
First, there is the solvent, the substance that dissolves the solute, and for our purposes,
the solvent is water, which is the most common solvent.
Solute is the substance that gets dissolved and solution is the mixture of the solvent
and the solute.
The more solute there is per amount of solvent, the more concentrated the solution gets.
Molarity is one way of expressing concentration.
How is molarity expressed?
Molarity is a measure of concentration.
Molarity is the number of moles of solute divided by liters of solution, expressed as
moles per liter, and symbolized with a capital M, sometimes italicized, which is pronounced
molar.
Molar.
Let's say we have a 1.25 molar solution of KCl, potassium chloride.
So 1.25 is our molarity.
1.25 molar KCl tells us there are 1.25 moles of KCl dissolved for every one liter of solution,
or 1.25 moles per liter.
1.25 molar represents 1.25 moles per liter.
Let's take a look at how we can use molarity mathematically.
First, we will look at how molarity is calculated and then use molarity in a calculation.
First problem, what is the molarity of 0.65 moles of sodium fluoride dissolved in a total
volume of 0.50 liters?
Molarity is moles per liter, so we divide 0.65 moles by 0.50 liters of solution, and
find the concentration to be 1.3 moles per liter, or 1.3 molar sodium fluoride.
The molarity is 1.3 molar.
Problem number two, in which we now have not moles, but grams.
What is the molarity of 25.2 grams of sodium fluoride dissolved in a total volume of 0.75 liters?
Since we have grams and not moles, the first step would to determine the number of moles
using the molar mass of sodium fluoride, which is 42.0 grams per mole.
Here, grams cancel, and now we have 0.60 moles sodium fluoride.
Molarity is moles per liter, so 0.60 moles divided by 0.75 liters gives us 0.8 molar
sodium fluoride.
Now, two problems to look at using molarity in a calculation.
Problem number three asks, how many moles of ammonia, NH3, would you need to dissolve
in a total volume of 2.5 liters in order to make a 1.4 molar solution?
Note that 1.4 molar is not a specific amount, it gives us the unit relationship between
moles and liters.
So we begin with 2.5 liters and multiply by the desired mole to liter relationship given.
Liters cancel and we are left with 3.5 moles of ammonia.
If you dissolve 3.5 moles of NH3 in enough water to make a 2.5 liter solution, you will
have a 1.4 molar solution of ammonia.
I hope you can see a calculation like this is very useful when you are trying to make
a solution of a specific volume and concentration.
Last problem, this time using mass.
Saline is a 0.154 molar sodium chloride solution.
What mass of sodium chloride is needed to make 4.5 liters of saline?
Note that here, as in the second problem, we are given mass rather than moles.
First step is to find moles of sodium chloride needed by multiplying the desired volume by
the molarity, which gives moles of sodium chloride.
But the problem asks for grams, so one more step to convert moles to mass using sodium
chloride's molar mass of 58.5 grams per mole.
The liters cancel, leaving us with moles.
The moles cancel, leaving us with grams.
Dissolving 40.5 grams of sodium chloride in enough water to make a 4.5 liter solution
results in a molarity of 0.154 moles per liter, or 0.154 molar sodium chloride.
Main points.
A solution is a mixture of solute and solvent.
Molarity is the ratio of solvent to solution volume.
So adding more solute increases the molarity.
If you remove the solute, you will be left with solvent.