Making Solutions
Making juice from a concentrate or juice crystals is an example of creating a homogeneous solution. If we were to make a glass of lemonade, we probably have a set amount of crystals we like to add to make it taste great. How much we may choose to add could differ from person to person as some way want a really strong lemon taste, while others not so much. Use this understanding to address the first question:
A take away from the above activity is the more crystals we add, the more concentrated the solution and the darker the yellow colour. Let’s now say you made yourself some lemonade but you found the taste too strong of lemon flavour so you decided to add more water to dilute it. In the activity below, drag and drop the glasses into the before and after box to see what has happened visually to your lemonade solution:
A key thing to realize in the above activity is that when diluting a solution we are NOT changing the amount of a species present we are just changing the overall volume that it is present in.
Calculating Concentrations
Using your understanding of molarity (M), let’s now add some numbers to the above lemonade problem. Use the idea that each triangle represents one mole of “lemonade crystals” to answer the following questions:
Great, now that this concept makes sense, lets add a calculation. Drag the correct molarity onto each glass (remember that each triangle represents a mole):
Dilutions
Making the lemonade is actually a dilution question. We start off with an initial solution concentration of our lemonade ( 7 mol/L or 7 M) and then add water and dilute the lemonade so the taste is less intense (5 mol/L or 5 M). Usually, we cannot see the number of moles on a molecular level like we can in this example here. Instead, we usually know an initial concentration and then use a formula called:
C1V1=C2V2 or CiVi=CfVf
where C = concentration (or molarity), V= volume, the “1” or “i” is initial or before dilution and the “2” or “f” is the final numbers after dilution.
How would you rearrange the dilution calculation above to solve for the final molarity (M)?
Let’s look at the above problem one more time and pretend we cannot see the number of triangles:
Up to this point we have looked at knowing the number of moles and the volume to determine the concentration/molarity. We can also do the reverse, where we know the molarity (concentration) and the volume and can use this to determine the number of moles.
Let’s think about what is happening with our units for the above question:
Whenever you are solving a numerical problem, try to visualize what is occurring, as this will help you realize when an equation might be set up incorrectly, as the answer may not make sense. Also remember to think about what is happening to your units – show every step with units and make sure they cancel, since wacky units might be the first sign of a calculation mistake.