The Value of Active Learning: Understanding Acidity Group 2 Sarah Malmquist Stony Brook University Joanna Kiryluk Stony Brook University Chandrika Narayan University of Massachusetts, Lowell Khaliliah Reddie University of Massachusetts, Lowell Valerie Purdie-Vaughns Columbia University Brent Stockwell Columbia University

Learning goals Learning objectives Appreciate active learning vs traditional teaching strategies Understand what pH is and be able to determine pH of sample Learning objectives You should be able to… Distinguish between active learning and traditional lectures Define pH Determine the pH of a solution using litmus paper Calculate the pH of a solution given the proton concentration

Setting the scene Problem to solve: readiness for active learning Who are you? Students in Bio 101 (or any introductory science class) First day of class What will you do? Participate in two activities

Part 1 Lecture: acidity

Acidity is a critical feature of biological systems Acidity is the concentration of H+ measured using pH pH is important living systems fish in water human blood tumor growth digestion

pH measures the concentration of protons in a solution The pH typically varies from 0 to 14 [H+] = concentration of H+ in Molarity (M = moles/Liter) pH = - log [H+]

How does the Log function work? Log 10 = Log (101) = 1 Log 100 = Log (102) = 2 Log 1000 = Log (103) = 3 Log 0.1 = Log (10-1) = -1 The log (base 10) of a number is the exponent the number 10 needs to be raised to in order to generate that number e.g. 0.001 M H+ = 10-3 M H+ = pH of 3 pH 3 = 10-3M

Acids have removable protons that dissolve in solution Lower pH = higher [H+] = acidic Lemon juice = pH 2 If [H+] = 10-7 M, pH = 7 = neutral water The combination of acidic or basic molecules dissolved in an aqueous solution imparts an acidity or basicity in that solution. The acidity or basicity of a solution is described by a number called its pH. By definition, the pH of a solution is equal to – log [H+]. Since in water protons actually become H3O+ cations, pH = –log [H3O+] in water. Therefore, a solution containing a proton concentration of 0.1 M has a pH of 1. As you can deduce, the greater the concentration of protons in solution, the lower the pH of that solution. Neutral aqueous solutions have a pH of 7.0. Acidic solutions such as lemon juice have a pH below 7, while basic solutions such as ammonia- containing window cleaners have a pH above 7. H2O H+ + OH- [H+] in water = 10-7

Solutions that differ by 1 pH unit have a ten fold difference in [H+] Lemon Juice pH = 2 0.01 M H+ Vinegar pH = 3 0.001 M H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ H+

Example of a pH calculation Add 1 mL 1M HCl (1 mol/liter) to 1 L water, pH drops from 7 to 3 Why? 1 mL of 1M HCl has 10-3 moles of H+ 10-7 mol (0.0000001 mol) in 1 L water (negligible by comparison) [H+] when added to 1 L = 10-3 moles/liter log[10-3] = -3 pH = -log[H+] = 3

Summary pH is a numerical scale for determining the acidity of a solution pH = - log[H+] It is important to know how to calculate changes in the pH of a solution

Clicker Question You have 5 mL of a solution of pH 4 and 5 mL of another solution of pH 7. Which of the following is the closest estimate for the resulting pH if you mix the two solutions together? 3.5 4.5 5.5 6.5 7.5

Part 2 Active Learning: acidity

A hands-on pH experiment GOAL: Understand how to measure and calculate pH 1. How to use a litmus paper to measure pH of a solution. Litmus paper: a strip of indicator paper that is dipped in a sample liquid. The paper turns a different color depending upon the pH of the solution. The strip is compared to a color standard to determine pH

A hands-on pH experiment 2. Measurements Use tube : “A” (5 mL of soda), “B” (5 mL of water), “C” (40 mL of water) Measurement 1: Take a strip of litmus paper and dip the end into a tube A with soda. Measure the pH with litmus paper, record the pH. Convert your measured pH to the [H+] in the sample and record it Measurement 2: Pour water from tube B to soda in tube A. Now you have diluted the original soda 2-fold. Use a strip of litmus to measure the pH of the diluted soda Record the pH, convert it to the [H+] in the sample, and record it Measurement 3: Pour water from tube C to tube A to further dilute the soda. Now you have diluted the original soda 10-fold. With a new strip of litmus paper, measure the pH.

A hands-on pH experiment pH = –log [H+] Soda volume (mL) Water volume Volume (soda+water) Fold dilution of soda pH [H+] (M) 1. 5 1 2. 10 3. 45 50 Please label each pH strip after you use it What was the pH change after a two-fold dilution? After a 10-fold dilution?

Clicker Question You have 5 mL of a solution of pH 4 and 5 mL of another solution of pH 7. Which of the following is the closest estimate for the resulting pH if you mix the two solutions together? (Talk with your whole table.) 3.5 4.5 5.5 6.5 7.5

Clicker Question You have 5 mL of a solution of pH 4 and 5 mL of another solution of pH 7. Which of the following is the closest estimate for the resulting pH if you mix the two solutions together? 3.5 4.5 5.5 6.5 7.5

SOLUTION [H+] = 10-4 M = 0.00010 M -log[10-4] = 4 = pH The number of protons in water is 10-7 M (0.0000001 M), which is negligible by comparison Two-fold dilution of the pH 4 solution [H+] = 5 x 10-5 = 0.00005 -log(5x10-5) = 4.3 = pH of mixture

Brainstorm How did the hands-on experiment help prepare you for the clicker question? Complements lecture Adds excitement

Decreased failure rate Freeman et al., 2014, Proceedings in the National Academy of Sciences

“I need the best possible grade in this course” Dream come true! Active learning in class Higher grades

Learning goals realized… You should now have… an appreciation for active learning compared to traditional teaching An understanding of what pH is and the ability to determine the pH of a solution

Thank you.

Results of clicker question after lecture

Results of clicker question after hands-on activity

Example of a pH calculation Supplemental handout Example of a pH calculation If you add 1 mL of 1M HCl (1 mol/liter) to 1 liter of water, the pH of the water drops from 7 to 3

Solution for people who “think in equations” (include physicists, mathematicians, etc) and want to be able to calculate pH in the most general case Concentration: pH:

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