Inquiry: Concentration and pH Of Lactic Acid

Purpose: To calculate the concentration, pH of the lactic acid in a Ring Pop.


  • Lactic Acid (Ring Pop)
  • Base: 0.10M NaOH
  • Indicator: Phenolphtalein (ptph)
  • Water (50.0mL)
  • Electronic Scale
  • Stir Stick
  • Mortar and Pestle
  • Pipette
  • Burette
  • 10mL Graduated Cylinder
  • 3x 100mL Beakers
  • 250mL Beaker


STEP 1: Crush Lactic Acid (Ring Pop) using Mortar and Pestle until it is powder or at least into tiny crystals

STEP 2: Place the crushed Lactic Acid on the electronic scale until you have exactly 10g

STEP 3: Place the 10g of crushed Lactic Acid into a beaker then add 50mL of water into said beaker. Stir the solution until all the Lactic Acid dissolves.

STEP 4: Grab 3 100mL beakers and add 3 drops of ptph in each beaker. Measure out 10mL of the Lactic Acid solution with a pipette and add it into each of the 3 beakers which contain the ptph.

STEP 5: Grab one of the 100mL beaker containing the Lactic Acid solution and the ptph and place it under a burette. Fill the burette with NaOH and titrate the solution until it changes color (you may have to shake the beaker a bit for the change to occur). When it changes color, note how much NaOH was used. Repeat process with the 2 other 100mL beaker.

STEP 6: Once you have titrated your 3 100mL beakers, clean up your station and continue with finding the rest of your data based on what you have achieved from this lab

Results & Calculations

NaOH used
Trial 1 7.5mL
Trial 2 7.4mL
Trial 3 7.2mL
Average 7.37mL



The actual of pH for lactic acid is 2.4. After completing the lab and doing all our calculations, we calculated a pH of 2.5 instead. It is only off by 0.1 so we weren’t off by much. Some possible errors for this cause could be some miscalculations with our data; not all of our Ring Pop dissolved and/or we may not have titrated the acid enough. Maybe if we added a little more of the [NaOH] into the acid, we may have gotten an exact pH.




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