Solubility is the amount of a substance that can be dissolved in a solvent. Solubility is typically discussed in terms of how *much solute* can be dissolved in a given amount of solvent, i.

Typically, solvents are waters solutions, but solvents can also be alcohols, glycols, and oils. The term “solvent” is very broad, **even including gases**!

When talking about solutions in chemistry, the word “solute” refers to the substance that is being dissolved. For example, salt (solvent: water) is the solute when discussing how much salt can be dissolved in water.

This article will discuss how to calculate the molarity of a solution that contains a given number of moles of solute in a given volume of solution.

## Calculate the molarity of the solution

The *last step* is to calculate the molarity, or concentration, of the solution. This is how many moles of solute are in **one liter** of solution.

You calculated the number of moles of copper in the **solid material** and added that amount to **two liters** of solution.

## Know your solutes

The second part of molarity is determining the number of moles of solute in a solution. This is what makes molarity special: It accounts for how many molecules or ions are in a given volume of solution.

As mentioned before, one molecule of glucose in **solution represents one solute**. To find the molarity of a solution that **contains six molecules** of **glucose per two liters** of solution, you would need to know the volume of the solution and how many molecules or ions are in that volume.

The difficulty comes in measuring the volume of a solute when you are dealing with an ionic compound. Ionic compounds behave differently when placed in water, so simply measuring the total volume by weight or by measurement does not work here. You have to measure the total volume by weight after removing the excess water.

## Know your volumes

Volumes can be calculated in a few different ways. The most common is by using a **graduated cylinder** to measure the volume of liquid.

You can also use a beaker or a jar to measure the volume of liquid, but this may not be as accurate unless you use the same size beaker or jar to measure the amount of solution.

Another way to *calculate solution volume* is by weighing the dissolved substance and pouring an equal amount of water onto it. Then, you would measure the amount of water it displaced and multiply that number by two. This *would give* you the solution volume.

Calculating molarity requires knowing the volume of the solution, which can be tricky if you are working with different types of containers.

## Calculate the moles of solute

Once you have determined the number of grams of the substance, you can calculate the moles of the substance. Moles is its own unit, similar to grams or liters.

To calculate moles, multiply the weight of the substance by the universal chemical Avogadro’s number (6.022 × 1023). This is a constant that defines a mole as a weight equal to that of a certain number of atoms.

So, if you had 2 liters of solution with 6 moles of solute in it, then you would have 2 L × 6 mol/L = 12 mol solute. The **solution contains 12 moles** of the substance per liter of solution.

## Divide the moles of solute by the volume of solution

So let’s go back to our original problem. You have a solution that *contains six moles* of glucose in two liters of solution. How many molarities is this solution?

You divide the number of moles of solute by the volume of solution, so you divide six by two. You get three as the answer, which means that the molarity of your solution is ** three mol**/L.

Three mol/L is also expressed as 1g/L, so your solution is 1 g/L glucose. You can measure the volume of your solution and calculate how much glucose is in it!

There are many situations where you need to know the concentration (molarity) of a solution, but one very important one is when **medical personnel need** to administer an appropriate dose of a medication via an IV drip.

## Use this equation to find molarity

Molarity refers to the number of moles of a solute per liter of solution. Molality refers to the number of moles of a solute per quantity of solution combined (usually water).

When calculating molarity or molality, you need to know the volume of the solution, how many molecules or ions are in that solution, and what element or compound it is. You also need to know the average mass of one molecule or ion.

For example, if you have 2 liters of a sodium chloride solution, with 6 molecules of *sodium per molecule* of chloride, your molarity would be: 6Na/Cl x 2L = 12 Na/L .

This is important to know when calculating solutions for experiments. If you are trying to find the concentration of a solution, you *must first determine whether* it is a solute or not. If it is a solute, then you must find the number of moles and multiply that by the liquid volume to find the total liquid volume.

## Check your work

Calculate the molarity of your solution by dividing the number of moles of solute by the volume of solution. Then multiply this number by a constant: 1 liter.

For example, say you had a solution that *contained 6 moles* of solute in 1 liter of solution. To find the molarity of the solution, *divide 6 moles* by 1 liter and then multiply by 1 liter again. You *would get 6 mol*/l, or 6 M!

Check your work by calculating the osmotic pressure of the solution. If you had a solvent that was not able to pass through a membrane, then its *osmotic pressure would* be equal to its molarity.

## Practice these types of problems

Solubility rules are very helpful in problem solving. There are solubility rules for **many different compounds**, such as salt, sugar, and alcohol.

Sodium chloride, also known as common salt, forms a saturated solution when it is mixed with water. This means that it dissolves completely into 2 liters of water.

Knowing this solubility rule is helpful when **solving molarity problems** that *involve sodium chloride*. By calculating the number of moles of sodium chloride in 2 liters of solution, you can determine the concentration of the solution.

The problem could then be solved by adding more water to find the concentration of the original solution.