Molarity
Tim Loftus
A number of lab-prepared solutions in the wastewater laboratory are measured in
molarity (M). It is one of the ways to measure the concentration of an element,
ion, or compound in solution. The potassium chloride solution used to calibrate
a conductivity meter and the standard sodium chloride solution for chloride
analysis are typically measured in molarity.
This unit of concentration is based on the chemical concept of a mole
(abbreviated to “mol” in most references). A mol is the gram-atomic (or
molecular) mass of an element (or molecule) and is a measure of the number of
atoms (or molecules). For example, from the periodic chart, the gram-atomic mass
of hydrogen is 1.0. One gram of hydrogen is one mol and contains 6.0 x 1023
hydrogen atoms. The gram-atomic mass of oxygen is 16. Sixteen grams of oxygen is
one mol and also contains 6.0 x 1023 oxygen atoms. Even though the masses are
different, one mol of each element contains the same number of atoms.
This same concept is also used to determine the mol of a compound. The
gram-molecular mass of water, H2O, is 18 (two hydrogen atoms at 1 and one oxygen
at 16 = 18). Eighteen grams of water is one mol and contains 6.0 x 1023 water
molecules.
Using the mol as a measurement tool is especially helpful when determining how
much of one compound or element reacts with another.
When the mol unit of measurement is used with compounds in solution, it is
called molarity. It is defined as the gram-molecular mass of a compound per
liter of solution. Take potassium chloride (KCl) for example. One mol of KCl is
equal to 74.6 grams (from the periodic chart, the gram-atomic mass of potassium
is 39.0 and that of chlorine is 35.5. Add together for the gram-molecular mass
of 74.6). If you took 74.6 grams of KCl and diluted this to one liter with
water, you would have a 1.00 M KCl solution. Double the grams to 149 and dilute
to one liter and you would have a 2.00 M KCl solution.
To determine how many grams of a compound you will need for a specific molarity,
use the following formula:
Grams of the compound needed = (M)(gram-molecular weight)(liters)
An example: The test method for conductivity requires that I standardize the
meter with a 0.010 M KCl solution. I don’t want to make a whole liter; 250 ml
would be enough. How many grams of KCl do I need to dilute to 250 mL to obtain a
0.010 M KCl solution?
Grams of the compound needed = (0.010 M)(74.6)(0.250 liters) = 0.187 grams KCl.
While you will find some of the chemical solutions in your laboratory measured
in molarity (M), most of the solutions will be measured in normality (N). These
will typically be your acids and bases. The concept of normality is similar to
molarity, and will be covered in the next article.
Please note that this article specifically covers what is typically found in a
wastewater treatment laboratory. There are exceptions to how the concentrations
of solutions are measured, and this depends on the scope and application of a
particular test method.
If you have any questions, suggestions, or comments, contact NEWEA Lab Practices
Committee Chair Tim Loftus at (508) 949-3865 timloftus@msn.com. For more
information on the NEWEA Laboratory Practices Committee, please contact Tim
Loftus or Elizabeth Cutone, NEWEA Executive Director, 100 Tower Office Park,
Woburn, MA 01801, (781) 939-0908, ecutone@newea.org.
All past articles are posted on our website. Go to www.NEWEA.org and follow the
link to the Committee Pages then to the Laboratory Practices page.
