Unit A: Chemical Change

Lesson 1.5: Calculating Concentration

Determining concentration qualitatively can give you an idea of the relative concentration of solutions. However, to determine the actual concentration of a solution, accurate measurements of amounts of solute and solvent have to be made.

Concentration is a comparison of solute to solvent, but there are a wide range of solvents and solutes, and the concentrations are vastly different.  Atmospheric methane, for example, is describe by saying 1800 out of every billion molecules in the atmosphere.  The concentration of chainsaw oil might be described as 150 ml per 5 gallons.  There are many ways to describe concentration.

In this section you will calculate concentration in three ways.  You will also learn how to make a calculation to convert a concentrate into a dilution.    

  1. calculate concentration in percent by volume
  2. calculate concentration in parts per million
  3. recall molar mass and converting between moles and mass.
  4. calculate volume in moles per liter (Molar concentration)
  5. define standard solution
  6. outline the steps in making a standard solution
  7. calculate solvent amounts using the dilution equation

Examine the table on page 40. 
You might be good at problem solving, but many students are not confident.  People who solve problems one step at a time, and show their work neatly, consistently get the right answer to numerical problems.  Please follow the following steps when problems solving. 

  1. Read the question carefully.
  2. Highlight or write down the known and unknown quantities.
  3. Choose the correct equations.  Its the one that has these quantities and only 1 unknown.
  4. Substitute the quantities carefully into the equation.  Show your units.
  5. Solve for the unknown using algebra steps.  Show more work if you are weak in algebra.
  6. Show your final answer with the correct number of significant digits.
Watch the video.  Just try to follow the steps.  Don't worry about understanding calculations.
    Percent by Volume is used when  a liquid solute is dissolved in a liquid solvent .  An example is the vinegar we buy at the store.

    The units for volume do not matter, but the units for volume must be the same!  You should see them cancel out while solving.

    Read page 41 and 42.  Work through the Examples with your calculator, making sure you know how to enter scientific notation (the tip on page 43 may be useful) and how to express the correct number of significant digits (reviewed on page 44) !

    Really do     the practice problems on page 42 and check your answers below  This is time well spent, especially if you are not a confident problem solver.

     

    Science 20 Textbook, Courtesy Alberta Education


    Parts per million (ppm) are used to express very small concentrations, like the amount of hormones that make you male or female, or the amount of toxins in the environment.

    The units for mass should be the same.  The will usually both be measured in grams.  

    • Read page 42 - 44.  Work through the example problems with your calculator!  Do the practice problems and check your answers below.  these are very important questions.  Be sure to do them! 

     

    Science 20 Textbook, Courtesy Alberta Education


    While Percent by volume and parts per million are the concentrations you will see the most in daily life,  molar concentration is the most common in chemistry laboratory calculations  because  most chemical reactions happen in solution . 

    Recall:  In theoretical chemistry (like balancing equations)   numbers of atoms  are used, but in practical chemistry  mass and volumes  are measured.  In Science 10 your learned to convert between these things using the expression n = m / M, where M is the molar mass. 
    • Review Example Problem 1.12a on page 45.  If you are not confident, try the questions below.

    The  units for molar concentration are mol / L.    

    • Read page 44.  Work through Example problems 1.11 and 1.12b.  Do practice problems 34 - 36 and check your answers below.
    Dilution happens when solvent is added to a solution.  It is important to understand that the amount of solute does not change in this process.  So the mass before equals the mass after and because m = C x V,  the equations C1V1 = C2V2 is derived.

    Read page 48 - 50.  Work through the example.  NOTE:  The volume calculate is the total volume.  A common mistake is to give this volume instead of giving the volume added, as in the example. 
    Do practice problems 37 and 39 (38 if you need more practice) and check your answers below.

     

    Science 20 Textbook, Courtesy Alberta Education


    Science 20 Textbook, Courtesy Alberta Education


    Standard Solutions

    A standard solution is a solution of very accurately known concentration that can be used to make other solutions.  The solutions are made from a solid (involving the molar concentration calculation) or a liquid (involving the dilution equation).  The standard solution is diluted to make solutions used in the lab (using the dilution equation).

    The procedure has many steps, but the main idea is to rinse many times to ensure all the solute goes into solution, and measure the volume accurately.  

    You might like to view the two videos below, especially if you do not have a lab.  The first is a review of this section and an overview of preparing a solution.  the second is how to prepare a solution in detail, and may assist you with one or two assignment questions.




    In the next investigation you will observe and practice the skills used for making a standard solution and a dilute solution. 

    • Read the entire investigation on pages 46 to 48 of the textbook.

    If you have access to a supervised laboratory, do Path 1.  If you do not have access to a supervised laboratory, Path 2.  This investigation is important to help you understand solutions and will assist with some assessment questions.

    • Follow the directions of the investigation.  Do Pre-Lab questions 1 and 2.  Check your work below.  Do analysis questions 3 - 7.  Check your answers below. 

      Science 20 Textbook, Courtesy Alberta Education

    For this path you will watch a lab technician use a pipette and volumetric flask to dilute a previously prepared solution of cobalt(II) nitrate. The concentration involved and the substance are not the same as those in the investigation, but the techniques are the same.

    • Complete questions 1 and 2 of “Pre-Lab Analysis” on page 47.
    • View the segment “Diluting the Solution”  and answer questions 3 to 7 on page 48. Check your answers in "Path 1"
    Solutions are often shipped in concentrated form to save shipping costs. If you need a solution of a particular concentration, you add distilled water to a calculated volume of concentrate.

    • Read pages 48 to 51 of the textbook, starting at “Diluting Solutions” and ending at “1.5 Summary”.  Do 1.5 Questions on page 52 # 1, 3 - 7, 9

      Science 20 Textbook, Courtesy Alberta Education


    Go to Assignment 1.5: Calculating Concentration.