Solution concentration is an important parameter in chemistry. It is typically defined as the number of moles of a constituent element or *compound per volume* of solution.

Most solutions are made up of water and a constituent substance, such as potassium chloride solution, which is made up of water and potassium chloride (KCl). Solution concentration is usually expressed in molarity, which is the number of moles of a **substance per liter** of solution.

Concentration can be expressed in any unit system, but only if a *standard unit system* is established. For example, one litre of **every substance containing one mole** of a constituent element or compound is one Molarity.

This article will discuss how to solve for the concentration (molarity) of a solution after being given other information such as the number of moles and the volume exchanged.

## Calculate the molarity of the solution

The molarity of a solution is the number of moles of a *substance per liter* of solution. If you had 2 liters of the KCl solution described in the What Is the Molarity of a Solution Containing 5.0 Moles of KCL in 2.0 L of Solution? section, then you would have 10 moles of KCl in your solution.

You can also refer to the molarity as concentration. A concentrated solution is one where there is a high number of molecules or *ions per unit volume*. The more molecules and ions that are in the solution, the higher the concentration is.

The formula for calculating molarity is n=n/V, where n=number of molecules or ions, V=volumeofsolution.

## Know your constants

Chemists use a series of constants to quickly determine the concentration of solutions. These include the gas constant, Avogadro’s number, and *solution concentration constants* for molarity, molality, and osmolarity.

The gas constant, or the **universal constant** for temperature (uCT), is used to determine molality or osmolarity. Molality is the concentration of solute in a solution expressed as * moles per liter* of solvent; osmolarity is the concentration of solute in a solution expressed as moles per liter of solution.

Concentration units for these are reciprocal quantities. For molality, the amount of solvent in a given volume is counted; for osmolarity, the amount of solvent is counted.

## Know your inputs

The first step in determining the molarity of a solution is knowing what ions are in the solution, how many moles of each ion are in the solution, and how many liters of solution are being analyzed.

In this case, you **would need** to know how many moles of potassium chloride (KCl) are in 5.0 moles of KCl dissolved in 2.0 liters of solution. The ratio of molecules is consistent between both substances, so that can be verified.

Then, you *would take* the number of moles of KCl in 5.0 moles and multiply that by the volume in liters to get the total volume of solution. This *would give* you an accurate representation of how many liters of *solution contain 5*.0 moles of KCl.

## Use math to solve for what you want

Now that you have determined the molarity of your solution, you can use that information to answer other chemistry questions. For example, you can determine how many moles of a different substance it *would take* to produce a solution of a certain molarity.

You can also use the molarity to calculate the volume of **solvent required** to make a certain volume of solution. Or, you could use the molarity to calculate the concentration of a solute in a given volume of solution.

As mentioned before, **one way** to think about molarity is as how many molecules are in one liter of solution. So, when **calculating molecular concentrations**, one way to think about it is in terms of how many molecules are in one liter of the solution.

## Check your work

Calculating molarity is a **pretty straightforward process**, but it is important to check your work to ensure you get the correct answer.

There are **two main ways** to check your molarity solution. The first is to calculate the volume of the solute using the calculated concentration and volume of solution, then calculate how many moles of solute that represents. They should be the same!

The second check is to calculate how many moles of solvent there are in the solution, then divide that by the known quantity of solute present. The **resultant number** should be close to the concentration of solution given.

Both of these checks can be done using a calculator with the *right functions*, or even just a simple calculator.