A Laboratory Experiment to Review Basic Chemistry Lab Techniques

showing blue precipitate of copper hydroxide forming in lighter blue solution

When the COVID pandemic of 2020/2021is eventually eradicated and more colleges resume in-person laboratory instruction again, many instructors will face the daunting reality that students entering upper-level chemistry classes may have little or no actual hands-on college lab experience having completed their general chemistry laboratory sequence through virtual simulations or video classes.

To bring students ‘up to speed’ on basic laboratory skills normally mastered during a general chemistry sequence, a simple laboratory experiment has been designed to address this impending problem. The experimental procedure is available in the Supporting Information as a set of stand-alone instructions that instructors can copy and distribute to students. You will also find teacher notes. (Log into your ChemEd X account to access the Supporting Information). 

This laboratory activity can be assigned to any upper-level class as an introductory experiment to provide students with a hands-on exercise that revises basic lab skills. This includes using apparatus such as the electronic balance, hotplate/stirrer, vacuum filtration, pipet, buret, volumetric flask, and bench spectrophotometer as well as practicing important techniques such as synthesis, filtration, dilution, and titration. The data collected during the experiment is used to complete basic but important mole, molarity, percent & theoretical yield, limiting & excess reactant, titration, and dilution calculations. Students will also regenerate the main reactant, copper(II) sulfate, which may be reused in future classes, minimizing reagent cost and illustrating the importance of lab waste recycling.

This experiment is a safe, economical, and convenient activity for instructors to re-introduce students to basic chemical apparatus and techniques they may have only experienced through virtual instruction. This will better prepare them to tackle upper-level in-person chemistry lab courses.

Concepts: 

stoichiometry, synthesis, laboratory instruction

Time required: 

If students watch and review the 28 minute video ahead of time, they should be able to complete the steps through parts A - D within 2 - 2 1/2 hours.

Materials: 

Chemicals

  • Copper(II) sulfate                               
  • Sodium hydroxide                             
  • 3 M sulfuric acid                                 
  • Potassium iodide                              
  • 3% Starch solution                            
  • Approx. 0.05 M sodium thiosulfate
  • Ethanol
  • Acetone

Apparatus 

  • Bench spectrophotometer and tubes
  • Hot-plate/magnetic stirrer and stirring bar
  • 100/150 mL beakers
  • 10 mL and 100 mL graduated cylinders
  • 25 mL and 50 mL volumetric flasks
  • Buchner funnel and filter paper to fit
  • 5 mL Pipet;  Buret
  • Wash bottle with deionized water
  • Vacuum filter flask
  • Thermometer
  • Oven at ~ 130℃

 

Background: 

The experiment is based on simple reactions involving copper compounds. Copper(II) sulfate is first reacted with sodium hydroxide yielding insoluble copper(II) hydroxide (figure 1):

          CuSO4 5H2O (aq)  +  2NaOH (aq)  →  Cu(OH)2 (s)  +  Na2SO4  (aq)  +  5H2O (l)        Equation 1

blue precipitate of copper hydroxide forming in lighter blue solution

Figure 1: Copper(II) hydroxide precipitate forming

 

After heating in aqueous solution, the hydroxide readily converts to copper(II) oxide:

                                   Cu(OH)2 (s) →   CuO (s)  + H2O (aq)                                                   Equation 2

With sulfuric acid, the CuO reforms aqueous copper(II) sulfate:

                         CuO (s) + H2SO4 (aq)  →  CuSO4 (aq)  +  H2O  (l)                                        Equation 3

Students complete the reactions described by equations 1-3, isolating the intermediate CuO by vacuum filtration then converting it back to copper(II) sulfate. By measuring the absorbances of the initial and reformed copper(II) sulfate solutions, the percent recovery of the copper can be estimated. The copper content is also determined by a standard iodine/starch titration with sodium thiosulfate. Finally, students gain experience with the pipet and volumetric flask by completing a dilution.

Video 1: , Online Chemistry YouTube Channel, May 7, 2021

 

The complete step-by-step lab instructions along with a materials list and teacher notes are provided in the Supporting Information. An accompanying video1 may be viewed by students prior to the lab. Familiarizing themselves with the apparatus and techniques beforehand will ensure the experiment can be completed in a single 2 - 2½ hour lab period. The experiment could also be used as a general introductory lab to review basic lab techniques for students beginning any upper-level course regardless of their previous lab experience.

Procedure: 

Part A: Preparation of Cu(OH)2 and conversion to CuO

Step 1: Weigh about 1.2 g of NaOH pellets into a 100 mL beaker (be sure to keep the lid on the NaOH bottle at all times when not in use). Record the accurate mass of sodium hydroxide on your data sheet. Add 10 mL (graduated cylinder) of deionized water. Set aside and swirl occasionally to dissolve the solid while proceeding to step 2.

Step 2: Weigh exactly 3.00 g of CuSO4∙5H2O into a 150 mL beaker (or onto a plastic weighing dish or weighing paper then transfer to a beaker).Record the mass of copper(II) sulfate on your data sheet. Add 40 mL of deionized water (graduated cylinder) and a magnetic stirring bar. Stir until the solid dissolves. (The solid does not need heating to dissolve, but the hotplate can be turned on to medium heat at this point for use in step 3). Transfer the copper(II) sulfate solution via a funnel to a 50 mL volumetric flask, rinsing the beaker with a little deionized water (~5 mL) adding the rinsing to the flask. Fill the flask to the mark using the wash bottle and shake to mix.

Step 3: Measure the absorbance of the solution at 650 nm with a bench spectrophotometer, using deionized water as the blank. Record the absorbance. 

Step 4: Transfer all the copper sulfate solution (from the volumetric flask and spectrophotometer tube) back into the 150 mL beaker, rinsing both the flask and tube with 1-2 mL of deionized water and adding to the beaker. Place the stirring bar back into the solution and place the beaker on the hotplate with rapid stirring and heating. Add the sodium hydroxide solution. A blue precipitate will form.

Step 5: Heat the solution to around 60oC for about 5 min or until all the blue color disappears and a black-brown solid forms. Remove the beaker from the hotplate.

Step 6: Collect the product by vacuum filtration using a Buchner funnel containing a filter paper moistened with a little water from a wash bottle. (When beginning the filtration, first turn on the vacuum so the moist filter paper grips the funnel then pour a little of the dark solution onto the center of the filter paper. Then continue to add more until all has been transferred to the funnel). Rinse any solid from the beaker into the funnel with the wash bottle, keeping the magnetic stirring bar in the beaker. The filtrate should be colorless. As the filtration nears completion and while the precipitate in the funnel is still quite moist, pour 15 mL of water into the funnel to wash the precipitate. When most of the water has washed through, remove the flask from the vacuum and discard the filtrate. Reconnect the vacuum and continue filtering by washing the dark precipitate with ethanol 3 times (3 x 5 mL). Wash an additional two times with 5 mL portions of acetone. Keep the vacuum attached until no more solvent washes through and the solid begins to dry. Discard the ethanol/acetone filtrate into an organic waste bottle.

Step 7: Place the funnel (labeled with your name) with the precipitate in a 130℃ oven for about 10-15 min until the dark solid is dry. (While waiting for the solid to dry in the oven, the reagents and apparatus for the titration in Part C can be collected and assembled, eg buret can be cleaned and filled with sodium thiosulfate solution). Remove the funnel from the oven using insulated gloves and allow to cool to room temperature.

Step 8: Weigh an empty, clean, dry 100 mL beaker. Record the mass of the empty beaker. Carefully scrape the dry dark solid from the filter paper into the beaker using a spatula. Any solid adhering to the funnel should also be scraped away and added to the beaker. (If any lumps of the solid still appear wet, the beaker can be placed in the oven for a few more minutes to completely dry, after which it is removed and cooled before weighing). Record the mass of the beaker plus dry product and determine the mass of dry product, CuO.

 

Part B: Conversion of CuO back to copper(II) sulfate

Step 9: Use a wash bottle to rinse (3-4 mL) any CuO left on filter paper or Buchner funnel into the beaker containing the weighed CuO (to ensure complete conversion back to CuSO4). Add 30 mL deionized water to the beaker, a stirring bar, and 5 mL of 3M sulfuric acid (graduated cylinder). Heat the solution at about 50℃ with stirring until it turns blue and all the CuO dissolves (~ 5 min). If needed, a few more drops of sulfuric acid can be added to speed up the reaction. Remove from the hotplate and cool slightly.

Step 10: With a small funnel, transfer the solution to a clean 50 mL volumetric flask (can be wet with water), rinsing out the beaker with a little deionized water and adding to the flask. Fill to mark.

Step 11. Pour some of the copper(II) sulfate solution into a dry spectrophotometer tube (or rinse a wet tube with a little of the solution) and measure its absorbance at the same wavelength used in step 3. Use deionized water as the blank. Record the absorbance. Pour the remaining copper(II) sulfate solution from the volumetric flask into a clean 100 mL beaker (add the portion in the spectrophotometer tube to the copper waste bottle provided by the instructor). The solution will be used in Parts C and D.

 

Part C: Titration of reformed copper(II) sulfate

Step 12: Pipet 5.00 mL of the reformed copper(II) sulfate solution in the beaker from step 11 into a 125 mL Erlenmeyer flask. Add 20 mL deionized water (graduated cylinder) followed by 1.50 g of potassium iodide and swirl to dissolve. A precipitate will form and the solution will turn brown.

Step 13: Titrate the copper solution with approx. 0.05 M sodium thiosulfate solution. Record the actual molarity. To perform the titration, first add about 5-10 mL of the sodium thiosulfate from the buret until the darker brown color lightens to an orange-brown. Add 2 mL (graduated cylinder) of 3% starch solution. The solution will turn a dark black-blue color. Continue titrating until one drop turns the solution white (there will also be a white precipitate). Record the total volume of sodium thiosulfate solution added, including the first addition prior to adding the starch). Titrations should normally be repeated several times for accuracy and if time permits your instructor may require this. Discard the titrated solution into an appropriate waste container.

 

Part D: Dilution of reformed copper(II) sulfate

Step 14: Pipet 5.0 mL of the reformed copper(II) sulfate solution from the beaker in step 11 to a 25 mL volumetric flask and dilute with water to the mark. Measure the absorbance of the diluted solution at the same wavelength used in step 3. Use deionized water as the blank. Record the absorbance of the diluted copper solution.

Step 15: Place all unused copper(II) sulfate solutions in a class recycle container.

Editor’s Note: Log into your ChemEd X account to access the Student Document and Teacher Notes. Don't have an account? Free registration is available .

Questions: 

1.  Calculate the molar masses of CuSO4∙5H2O,  NaOH, Cu(OH)2, and CuO.

      The balanced equations for the formation of CuO are:

        CuSO4∙5H2O (aq)  +  2NaOH (aq)  →  Cu(OH)2 (s)  +  Na2SO4  (aq)  +  5H2O (l)        Equation 1

        Cu(OH)2 (s) →   CuO (s)  + H2O (aq)                                                                            Equation 2

2.  Determine which of the reactants is limiting and which is in excess in equation 1.

3.  Calculate the theoretical yield of CuO based on the mass of the limiting reactant.

4.  Calculate the % yield of CuO. If it is greater than 100%, what does this most likely mean?

5.  Calculate the molarity of the copper(II) sulfate solution in the original 50 mL volumetric flask.

6.  According to Beer’s Law, A = εbC, meaning that the concentration (C, molarity) of a solution is directly proportional to its absorbance (A). Provided ε and b remain constant for two solutions and the absorbance and concentration of one solution are known, the concentration of a second solution can be estimated.

Compare the absorbance of the original copper(II) sulfate solution with the absorbance of the reformed solution. What does this tell you about the molarity of the reformed copper(II) solution and hence the recovery of the copper(II) sulfate?

7.  The determination of copper by titration is a standard volumetric analysis using iodine/thiosulfate and a starch indicator. The equations for the reactions are:

                                           2Cu2+ + 4I-   2CuI(s) + I2

                                           I2 + 2S2O32-  2I- + S4O62-

Use the titration data to calculate the molarity of the reformed copper(II) sulfate solution. How does this compare with the molarity calculated in question 5?

8.  Use M1V= M2Vto determine the concentration of the diluted solution.

9.  Use Beer’s Law to determine the concentration of the diluted solution.

10.  How do the answers to question 8 and question 9 compare?

Preparation: 

Share the video and student document with students in advance.

Set up the equipment and chemicals.

Safety

General Safety

For Laboratory Work: Please refer to the ACS .  

For Demonstrations: Please refer to the ACS Division of Chemical Education .

Other Safety resources

: Recognize hazards; Assess the risks of hazards; Minimize the risks of hazards; Prepare for emergencies