READ: Concentration

Concentration (the measure of how much of a given substance is mixed with another substance) is one way that chemists describe solutions. Solutions can be said to be dilute or concentrated. A concentrated solution (one in which there is a large amount of solute in a given amount of solvent) has more particles dissolved in it than a dilute solution (one in which there is a small amount of solute in a given amount of solvent). A dilute solution is a concentrated solution that has been diluted or watered down. Think of the juice containers you buy in the grocery store. In order to make juice, you mix the frozen juice from inside these containers with about 3 or 4 times the amount of water. Therefore, you are diluting the concentrated juice. The terms “concentrated” and “dilute,” however, only provide a qualitative way of describing concentration. In this chapter, we will explore some quantitative methods of expressing solution concentration.

In this image, the solution on the left is more concentrated (more solute particles compared to solvent particles) than the solution on the right.

As you move from left to right, the solutions become more concentrated.

Molarity

Of all the quantitative measures of concentration, molarity –(the number of moles of solute per liter of solution) is the one used most frequently by chemists. The symbol given for molarity is M, or moles/liter.

Molarity(M)=moles of soluteliters of solution

Example 1

A chemist wants to make a 2M solution of salt water. How could they do it?

Solution:

2M NaCl=2 moles of NaCl1 liter of solution

  1. First find the molar mass of NaCl using a periodic table.
  2. Next, multiply the molar mass by 2 since the solution asks for 2 moles.
  3. Lastly, weigh out the 2 moles of NaCl then add water until the total solution reaches a volume of 1 liter.

Molarity is very easy to calculate when making 1 liter of solution, but often times chemists want to make more or less of a solution that has the same concentration. What if you want to make 2 liters of 2M NaCl (the same concentration as Example 1)? Since you are adding twice as much water, you would have to add twice as much NaCl. What if you want to make 0.5 L of 2M NaCl? 

Example 2

What is the concentration, in mol/L, when 2.34 moles of NaCl has been dissolved in 500.0 mL of H2O?

Solution:

The concentration of the NaCl solution is 4.68 mol/L or 4.68 M.

[NaCl]=2.34 mol0.500 liter=4.68 M

Example 3

A solution is prepared by dissolving 42.23 g of NH4Cl into enough water to make 500.0 mL of solution. Calculate its molarity.

Step 1: List the known quantities and plan the problem.

Known: mass = 42.23 g NH4Cl; molar mass NH4Cl = 53.50 g/mol; volume solution = 500.0 mL = 0.5000 L

Unknown: molarity = ? M

The mass of the ammonium chloride is first converted to moles. Then the molarity is calculated by dividing by liters. Note the given volume has been converted to liters.

Step 2: Solve.

42.23 g NH4Cl×(1 mol NH4Cl)(53.50 g NH4Cl)=0.7893 mol NH4Cl0.500 L==0.7893 mol NH4Cl1.579 M NH4Cl

Step 3: Think about your result.

The molarity is 1.579 M, meaning that a liter of the solution would contain 1.579 mol NH4Cl.

Molality

There are other units of concentration, including molality –(the ratio of moles solute per kilogram of solvent). Molality has the symbol m.

molality (m)=mol of solutekg of solvent

Molarity, if you recall, is the number of moles of solute per liter of solution. Volume is temperature dependent. As the temperature rises, the molarity of the solution will actually decrease slightly because the volume will increase slightly. Molality does not involve volume, and mass is not temperature dependent. Thus, there is a slight advantage to using molality over molarity when temperatures move away from standard conditions.

Example 4

Calculate the molality of a solution of hydrochloric acid where 0.2 mol of hydrochloric acid has been dissolved in 2 kg of water.

Solution:

m=0.2 moles of HCl2 kg of H2O=0.1 molality HCl

Parts Per Million

Parts per million, (ppm), (the parts of solute per 1 million parts of solution) is another unit for concentration. This unit is generally used to communicate really small concentrations. You already know that 1% means that there is one part out of 100 and that 2% means that there are 2 parts out of 100. Similarly, if you have 1 ppm that means that there is 1 part out of 1 millions parts and if you have 2 ppm there is 2 parts out of 1 million parts. PPM is used most frequently when dealing with environmental issues. You may have heard about parts per million when scientists are referring to drinking water or poisons in fish and other food products. To calculate parts per million, the following formula is used.

ppm=mass of solutemass of solution × 106

Example 5

Mercury is very toxic even at small concentrations. Mercury levels in fish have often been at the forefront of the news for people who love to eat fresh fish. Salmon, for instance, contains 0.01 ppm compared to shark which contains 0.99 ppm. In the United States, canned tuna is the most popular selling fish and has a mercury level of 0.12 ppm, according to the FDA statistics. This means that in 1000 g of tuna, there would be only 0.00012 g of mercury.  In shark, however, in the same 1000 g amount, you would have 0.00099 g of mercury. Although this seems like a very small amount, some toxins like mercury are dangerous even at these low quantities. 

Solution:

ppmmass of solutemass of solute===mass of solutemass of solution × 106(mass of solution)(ppm)1×106(1000.g)(0.12)1×106=1.2×10−4g

Summary

  • Concentration is the ratio of how much of a given substance is mixed with another substance.
  • Molarity is calculated by dividing the number of moles of solute by the liters of solution.
  • Molality is calculated by dividing the number of moles of solute by the kilograms of solvent. It is less common than molarity but more accurate because of its lack of dependence on temperature.
  • Parts per million means is the mass of solute per mass of solution multiplied by 1 million and is frequently used for environmental issues.
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Last modified: Thursday, 28 July 2016, 12:40 PM