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Chemistry 101: Molarity

Chemistry 101: Molarity

What Is a Mole?

A mole, abbreviated as “mol,” is a unit of measurement defined as the quantity of a substance that has the same number of particles found in 12.000 grams of carbon-12. This number, Avogadro's number, is 6.022 x 1023. Basically, Avogadro's number is used because it gives us a consistent method for converting between atoms/molecules and grams, making it easier for chemists to work with very large numbers of items, which occurs frequently in chemistry when measuring in grams wouldn’t make sense.

How Do You Calculate Molarity?

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Molarity is an expression of a solution’s concentration. The component of a solution that is present in the largest amount is known as the solvent. A chemical substance mixed in the solvent is called a solute, and solutes can be gases, liquids, or solids. For example, in the metal bronze, a solid (tin) is dissolved in another solid (copper). However, matter in any state can be the solute or solvent in a solution. In a saltwater solution, a solid (salt) is dissolved in a liquid (water).

To calculate the molarity of a solution, the number of moles of solute must be divided by the total liters of solution. If the amount of solute is given in grams, we must first calculate the number of moles of solute using the solute’s molar mass, then calculate the molarity using the number of moles and total volume. To calculate the molarity of a solution, we divide the moles of solute by the volume of the solution expressed in liters. The units of molarity are M or mol/L. A 1 M solution is said to be “one molar.”

Therefore, the equation determining molarity is as follows:

M = moles solute / liters solution

Check out this website for sample problems and other resources.

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How Do We Use Molarity in the Laboratory or Classroom?

Molarity is one of the most common units used to measure the concentration of a solution. Knowing the molarity of a solution is useful because by knowing it you not only know if it is diluted or concentrated but also the actual concentration. In a lab, a chemist must frequently prepare a given volume of solutions of a known molarity. The task is to calculate the mass of the solute that is necessary to reach the desired molarity. Chemists can use the molarity equation to solve for moles, and the result can then be converted to grams.

A variety of glassware is used to prepare solutions that can then be used to complete tests and experiments involving molarity. Among that glassware, graduated cylinders, volumetric flasks, volumetric pipets, and burets are often used to measure out specific volumes. Check out our recent Chemistry 101 article on how to measure liquids for science activities to learn more about the basics of effectively measuring liquids as you engage in scientific experimentation.

In the classroom setting, kits such as this molarity lab investigation kit offers students an introduction to the concept of molarity. Students can first try to make a solution with a specific molarity, demonstrating the importance of good laboratory technique. In the second set of experiments students can perform a titration on a known acid solution before using the concepts they have learned to identify the concentrations in three unknown solutions. 

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