Solution Concentration — Definition & Overview

Concentration is a measurement of how much solute there is per volume of solution. In chemistry, concentration is most often expressed in molarity by units of molar ($\text{M}$), which is moles per liter solution.

For example, lets say we dissolve 2 moles of sugar in 4 liters of water. The concentration of our solution would be as follows, $$\begin{align}C&=\frac{2\text{ mol}}{4\text{ L}} \\ &=.5\frac{\text{mol}}{\text{L}} \\ &=.5\text{ M} \end{align}$$

As we can see, this solution has a concentration of $.5$ molar. An important note is that in this calculation we assumed the sugar would have a negligible affect on the overall volume. In many cases this is an appropriate assumption to make. In most cases this assumption will result in a calculated molarity slightly higher than the actual molarity. For our purposes it is safe to assume that the solution volume is the same as the solvent volume unless otherwise stated.

Practice:

14.61 grams of $\text{NaCl}$ (molar mass is 58.44 g/mol) are dissolved in 381 milliliters of water (molar mass is 16.01 g/mol). What is the molarity of the solution?

To find this concentration we first need to turn grams of $\text{NaCl}$ into moles. $$14.61 \text{ g NaCl} \cdot \frac{\text{mol}}{58.44\text{ g}}=0.25 \text{ mol NaCl}$$ We can now divide by the volume of water in liters. $$C=\frac{0.25 \text{ mol}}{0.381\text{ L}}=0.656 \text{ M}$$ Thus, our solution has a concentration of 0.656 molar.

Sometimes concentration is denoted by square brackets around the solute, for example, [$\text{NaCl}$]. This notation is commonly used in equations that are reliant on concentration. For example, the pH of a solution is given by the equation below, $$pH = -log_{10}[H^{+}]$$ This can be read as, "the pH is the negative logarithm of the concentration of hydrogen ions expressed in molarity."

Concentration is the amount of solute that is added to a solvent. As I'm sure you've learned by now, a solvent is a substance that enables the solute to dissolve. For example, in salt water, salt would be the solute and water would be the solvent. Concentration is usually written in percentages or moles per liter (mol/L). Usually, you'll see the term molarity to describe the concentration in moles per liter.

Here are the main formulas you'll need to know dealing with concentration:

Remember that when you plug in numbers for these formulas, the units for the solute and solvent have to be equal!

$\text{molarity (M)} = \frac{\text{moles of solute}}{\text{liters of solution}}$

$\text{molality (m)} = \frac{\text{moles of solute}}{\text{kilograms of solvent}}$

(Video) Concentration Formula & Calculations | Chemical Calculations | Chemistry | Fuse School

by FuseSchool - Global Education

Concentration is a measure of how many particles of solute there are in a given volume of solution. In this video, FuseSchool shows you a neat trick for solving concentration problems.

Solution Concentration: How to determine a solution's concentration?