Making Solutions and Measuring Their Concentrations

March                                                                                                                                                                   Chemistry

Overview:  You will make a solution of know concentration in this lab by measuring the various components of the solution.  In particular, you will make 100 mL of copper (II) sulfate solution.  One striking property of this solution is its blue color.  This color can be used to determine the concentration of the solution.  How is this possible?  Well, the color of the solution is perceived because the hydrated copper ion in the solution absorbs some of the colors of the spectrum of the white light that passes through the solution.  If the solution appears blue, what color(s) are being absorbed?  (Remember from Conceptual Physics.)  Each lab group will make a solution of known concentration that is different from each other.  We will then place these solutions in a colorimeter.  This colorimeter measures the amount of light of a certain color (wavelength) that the solution absorbs.  The more concentrated the copper (II) ion the more light is absorbed.  A graph of Absorbance vs. [Cu²⁺] will be generated from the class data.  This graph is called a “Beer’s Law graph.”  Beer’s Law says that the absorbance of light passing through a colored solution is directly proportional to the concentration of the light absorbing species in the solution.  (If the relationship is directly proportional, what shape will the collection of data points have?)  Once we have generated our standard Beer’s Law Graph we will use this graph to determine the concentration of solutions which contain an unknown amount of Cu²⁺.

Equipment:

You will need the following items:

• 100 mL volumetric flask

• sample of CuSO₄-5H₂O (s)

• weighing scale

• weighing boat

• spatula

• distilled water

• cuvet

• cuvet wipes

Procedure: (Record data and observations on your separate sheet of paper designated for your lab write-up.)

1. Make a 100 mL solution of CuSO₄ of the concentration assigned to you by your instructor. 

2. First, calculate how many grams of CuSO₄-5H₂O (s) you need to add to the 100 mL volumetric flask. (Clearly, show your calculations and results on your lab paper.)

3. Add water to dissolve the salt, be sure that you wait to top off the volumetric flask until all of your salt is dissolved.  (Use the etched mark on the stem of the flask as the indicator of the level to which you need to fill.) 

4. Pour a small amount of your sample into a cuvet filling it ¾ full.  Be sure that there are no finger prints on the clear sides of the cuvet. 

5. Bring the cuvet to your instructor at the computers at the back of the room and measure the absorbance of your solution. 

6. Pour out the contents of your cuvet into the sink and rinse it with distilled water.

7. Obtain a copy of the Beer’s Law graph for the class and attach it to your lab sheet.

8. Read the absorbance of the solution of unknown concentration and record this as part of your data.

Worksheet for: Making and Measuring Solutions Exercise    NAME:______________

Analysis:

1.      Indicate the concentration of CuSO₄ that you and your lab group made: _________.

2.      Show the calculation for determining the mass in grams of CuSO₄-5H₂O (s) you need for your 100mL solution.

3.      Using the class generated “Beer’s Law Graph” shown on the Smartboard™, and given an unknown solution with an absorbance of 0.300, determine the [CuSO₄] of this unknown solution?

How many grams of CuSO₄-5H₂O (s) would have been used to prepare 100. mL of this solution?

Questions: (Answer these in the space provided.)

1.      Give two possible experimental errors that could have occurred during the course of preparing your solution and measuring the absorbance.  Did either of these happen to your group?

2.      What could/did you do to make it easier/faster to dissolve your copper(II) sulfate?  Explain this in terms of factors that affect the rate of dissolving.

3.      How would the degree of the color (darker or lighter) be different if:

(a)    a student used the same number of grams of anhydrous CuSO₄ (s) as they had calculated for CuSO₄ - 5H₂O?

(b)   a student added the determined amount of CuSO₄ - 5H₂O to exactly 100.0 mL, or a volumetric flask filled with water to the mark? 

4.      Briefly and in order, state the procedural steps, involved in making 250. mL of a 0.200 M NaCl solution.  (Give the specific mass of the salt involved.)