Molarity is how chemists measure the concentration of a solution, allowing them to relate concentrations to one another when calculating chemical reactions and working with chemical solutions. A concentration is what chemists use to refer to the amount of substance dissolved into a given amount of solution.
“The beauty of chemistry is that I can design my own molecular world.” — Ben L. Feringa
Molarity refers to the number of moles within a solution, and when chemical reactants combine in ratios of whole numbers their volume is expressed in moles. As a simple example, water’s chemical formula is H2O. Two moles of water can be combined with 1 oxygen mole to create 2H2 + O2, or two moles of H2O.
Of Solutions And Mixtures
When learning chemistry, you frequently deal with pure substances. Yet in the real world, many substances are mixtures of different chemicals. These chemical compounds are referred to as two different categories. If the different substances are mixed together and they produce a mixture that is uniform throughout the entire sample, it is referred to as a homogeneous mixture. By contrast, heterogeneous mixtures are the mixtures that are different in composition in different areas of the mixture.
Homogeneous mixtures are also referred to as solutions. Solutions can be in different phases: gas, liquid, or solid. Chemists often need to be able to determine the amount of a chemical or compound within a specific solution, which is referred to as the concentration of that chemical or compound.
One mole of a substance is defined as a number of molecules or atoms equal to 6.022 x (10^23). This is known as Avogadro’s number. Avogadro’s number is based on the number of atoms found within 12 g of C12 – a carbon isotope. Avogadro’s number is extremely helpful to chemists because it enables them to easily relate different weights of 1 mole of different chemical compounds.
The term “Avogadro’s number” was used by a French physicist (Jean Baptiste Perrin) in reference to his (Perrin’s) estimate of the number of particles in a substance. Avogadro himself was a lawyer with an intense interest in physics and math who eventually became the first physics professor in Italy. Josef Loschmidt was actually the first person to give an estimate of the number of particles in a specific amount of substance.
“Chemistry is necessarily an experimental science: its conclusions are drawn from data, and its principles supported by evidence from facts.” — Michael Faraday
At one point in time chemists frequently defined the concentration of a solution as the weight of the solute divided by the volume. Yet the mole is the most common way of expressing concentrations now, thanks to its consistency and the ease of conversion. To find the concentration of a solution (the molarity), you use the following formula:
Concentration = number of moles/volume.
Units of molarity are given as “moles per cubic decimeter“, usually represented as “M”. They’re also often written out as mol dm^-3. The liter is identical to the cubic decimeter, and sometimes you may see concentrations given as moles per liter or mol^-1.
Solutes Vs. Solvents
When it comes to the molar concentration of the solution, the chemical compound that makes up the largest percentage of the solution is known as the solvent. Any of the chemicals mixed with the solvent are referred to as solutes. Solutes can be solids, liquids, or gases. As an example, the atmosphere is approximately 78% nitrogen, making it the solvent. Meanwhile, it is approximately 21% oxygen and 1% carbon dioxide and other gases. These other gases are solutes.
The concentration or molarity of a given solute has the following definition: the number of moles of solute per given liter of solution.
To put that another way: molarity = mol solute/L of solution. The molarity of a solution has units. These units are given as mol/liter, which is typically abbreviated as just “M”. The molar concentration of a solute is frequently represented with brackets around the solute’s chemical formula, like this: [Cl-]. [Cl-] is the number of chloride ions in a solution. Knowing the molar concentration of a solution allows chemists to convert between the solute’s moles (mass) and the solution’s volume.
To sum up, the molar concentration or molarity of a substance is the number of moles within that substance when dissolved in 1 liter of solution.
Let’s look at some specific examples of this:
If one had a solution made out of 100 grams of sodium chloride in 2.5 liters of solution and wanted to find out the molarity of that solution, they could start by determining the “formula weight” of sodium chloride.
The formula weight is calculated by adding the atomic weights of the elements together. Sodium chloride is NaCl, so together the atomic weights of its elements are: 35.45 + 22.99 = 58.44. So there’s 58.44g of NaCl per mole.
With the amount of NaCl per mole known, the number of moles in 100 grams of NaCl can be determined simply by using the formula weight and dividing it into 100g of NaCl. Doing this calculation gives you approximately 1.71 moles of NaCl.
The molarity of the solution is now easy to calculate, just divide the amount of NaCl moles by the solution’s volume: 1.71 moles of NaCl divided by 2.5 liters of the solution equals 0.684 Molarity.
“Every chemical reaction has a transition state.” — Derek Barton
Here’s another example:
Here’s how we could find out the molar concentration of a solution made out of 2.355g of dissolved sulfuric acid into water. The solution’s total volume is 50.0 mL.
Sulfuric acid is H2SO4, so let’s start by converting the solute’s mass to moles via determining the molecular weight of sulfuric acid. The molecular weight of sulfuric acid is 98.08 g/mol.
We take the 2.355 grams of H2SO4 and multiply it by the result of 1 mol/98.08. This gives us 0.02401 mol of H2SO4.
To finish finding the molar concentration of the solution, the total volume of the solution and the number of moles of sulfuric acid just need to be put into the formula to find the molar concentration.
Formula: Mol solute/L of solution = H2SO4. 0.02401 mol/0.050 L = 0.48 M.
- Homogeneous mixtures are those with uniform compositions – also called solutions.
- Heterogeneous mixtures have non-uniform composition.
- Solutes are chemicals that represent the smaller amounts of the mixture, while the solvent is the chemical that makes up the largest amount of the mixture.
- Molar concentration is determined by the following formula: Molarity = mol solute/L of solution.