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Acids and Bases

Investigation 2 – Lab

 

 

 

 

 

 

 

MINDSET

This Investigation is designed to:

  • allow you to view and demonstrate the concept of concentration,
  • introduce you to the concept of decreasing the concentration of a solution by dilution,
  • give you the opportunity to perform a serial dilution of an acid,
  • allow you to measure the pH of various concentrations of an acid, and
  • allow you to correlate pH with the concentration of hydrogen ions.

BE PREPARED

Student Preparation for the Investigation includes gathering the following materials.

Note: The materials are listed in your SDR. They are also listed below for your reference.

  • (1) pair of goggles per student
  • (1) pair of gloves per student
  • (1) test tube rack
  • (6) 15 ml centrifuge tubes
  • (6) dropper pipettes
  • (1) box of pH paper strips
  • (1) pair of forceps
  • (1) lab marker
  • (1) piece of plastic wrap 30 cm long, masking tape
  • (1) 400 ml beaker filled with distilled water
  • (1) 50 ml beaker containing 10ml of 1N HCl colored with red food coloring

Direct one student from each lab group to collect the materials listed in their SDRs.

INVESTIGATE

  • Dilution is accomplished when additional solvent but no additional solute is added to a solution. The result is a less concentrated solution than the original solution.

Note: The procedural steps are listed below for your reference.

  1. In this Investigation, you will use a technique called serial dilution to dilute an acid 5 times. You will determine the H+ ion concentration each time by measuring the pH of each of the solutions.
  2. Label the 50 ml beaker “HCl” and the 400 ml beaker “H2O.”
  3. Place the centrifuge tubes in a row in the test tube rack. Using the lab marker and the masking tape, label the left most centrifuge tube “HCl.” Label the next centrifuge tube “1,” the next “2,” and continue until you have labeled the last centrifuge tube “5.”

Acids 2 Lab Test Tubes

  1. Use one of the dropper pipettes and fill the “HCl” centrifuge tube with 7 ml of hydrochloric acid (HCl) from the 50 ml “HCl” beaker. Screw the cap back on the centrifuge tube and return the dropper pipette to the beaker.
  2. Use a second dropper pipette to add 9ml of distilled water from the 400 ml “H2O” beaker to centrifuge tubes 1, 2, 3, 4, and 5. Add water until the volume reaches the 9ml mark on the side of each centrifuge tube. Screw the caps back on the centrifuge tubes and return the dropper pipette to the beaker.
  3. Use the following diagram to help you with your serial dilutions.

Acids 2 Lab Dilution Diagram

Note: Hydrochloric acid (HCl) is not normally red. Red food coloring has been added to help you see the differences among your dilutions.

    1. Completely empty the dropper pipette from the 50 ml “HCl” beaker into the beaker. Use the dropper pipette to transfer 1ml of the HCl from the centrifuge tube labeled “HCl” to tube 1. The volume should reach the 10 ml mark on the side of the tube.
    2. Return all excess hydrochloric acid (HCl) in the dropper pipette to the centrifuge tube labeled “HCl” and screw the cap on. Return the dropper pipette to the 50 ml “HCl” beaker.
    3. Screw the cap back onto tube 1 and turn it upside down then upright again three times to mix the contents.
    4. Use a clean dropper pipette to transfer 1ml of the solution from tube 1 to tube 2. The volume should reach the 10 ml mark on the side of the tube. Return any excess solution to tube 1 and screw the cap on.
    5. Screw the cap back onto tube 2 and turn it upside down then upright again three times to mix the contents.
    6. Repeat this procedure as shown in the diagram until the dilutions have been prepared for tubes 3, 4, and 5.
  1. Spread the plastic wrap on the table next to the test tube rack.
  2. Label the top of a pH paper strip with “HCl.” Label 5 additional strips with numbers 1, 2, 3, 4, and 5.

Acids 2 Lab pH Strips

  1. Using forceps grasp the top of the pH paper strip labeled “HCl.” Dip ¼ of the strip into the liquid in the tube labeled “HCl.”
  2. Place the pH paper strip just inside the lower-left corner of the plastic wrap.
  3. Dip ¼ of strip 1 into the liquid in tube 1.
  4. Remove the pH paper strip and place it to the right of the strip labeled “HCl.”
  5. Dip ¼ of strip 2 into the liquid in tube 2. Remove the pH paper strip and place it to the right of strip 1.
  6. Repeat this procedure until you have lined up all 6 pH paper strips on the plastic wrap as shown in the picture. Fold the excess plastic wrap down over the strips.
  7. Starting with the pH paper strip numbered 0, match the color on the strip with the pictures of the squares on the pH chart. Estimate the pH of the HCl that most closely corresponds to the color of the colored squares in the pictures. Find the pH to the nearest half of a pH unit, for example, 3.0, 3.5, or 4.0.
  8. Record the pH value in Table A.
  9. Find the pH of the solutions in tubes 1, 2, 3, 4, and 5 and record the pH values in Table A

CLEAN UP

Be sure to clean up your lab bench when you have completed your experiments.