Molarity and dilutions

Molarity is a way of quantifying the concentration of a solution.

Dilution is a way of decreasing the concentration of a solution.

Both molarity and dilution are essential concepts for correctly performing chemical experiments in a laboratory.

Let’s say that we have two glasses of water.

In one glass, we put a small amount of sodium chloride which is table salt.

We can assume that all of this sodium chloride dissolves into the water to form an aqueous solution.

In our other glass, we put a much larger amount of sodium chloride which we will also assume completely dissolves in the water.

Because the first glass has less sodium chloride in the solution, we would say that this glass contains a more dilute solution of sodium chloride.

Because the second glass has more sodium chloride, we would say that this glass has a more concentrated solution of sodium chloride.

The words dilute and concentrated allow us to compare our two solutions of sodium chloride; however, they don’t quantify exactly how much more concentrated one solution is compared to another.

When we are quantifying the concentration of a solution, we need to use the concept of molarity.

The definition of molarity is moles of solute divided by the total volume of the solution in Liters.

Let’s use the definition of Molarity to make a solution of sodium hydroxide.

Suppose we want to make 500. milliliters of a 1.00 M solution of sodium hydroxide.

So, we know that the definition of molarity is moles of solute divided by liters of solution.

We plug 1.00 molar into our equation for the molarity.

We also plug in our desired quantity of the final solution, which is 500. mL.

But because the definition of Molarity is moles per liter, we need to convert 500. mL to 0.500 L.

So the equation for molarity is that x divided by 0.500 L equals 1.00 molar, where “x” is the number of moles of sodium hydroxide we want in our solution.

Solving for “x”, we can see that we want to use 0.500 moles of sodium hydroxide to make our solution.

But how many grams of sodium hydroxide does this correspond to?

We now need to convert moles to grams.

To do this with any substance, we always need to use the molar mass.

The molar mass of sodium hydroxide is 40.0 grams per mole.

Since we need 0.500 moles of sodium hydroxide to make our solution, we can multiply 0.500 moles times 40.0 grams per mole.

The “moles” cancel out, and we end up with 20.0 grams of sodium hydroxide.

So to prepare 500. milliliters of a 1.00 molar solution of sodium hydroxide, we would dissolve 20.0 grams of sodium hydroxide in enough distilled water to get 500mL for our total solution.

Let’s talk about how to make 500. mL of our 1.00 Molar solution of sodium hydroxide in the laboratory.

Our first step is to mass out 20.0 grams of sodium hydroxide using a balance.

Then we add the sodium hydroxide to a 500. mL volumetric flask.