The unit of acids and bases is difficult for most students in Advanced Placement Chemistry. The variety of various calculations can be overwhelming. I decided it was time to make the pH calculations more exciting. After completing notes and examples on each type of solution throughout the unit, the students in my AP class were given a handout to organize the calculations. The students completed the "Student Notes for solving pH problems" (download a copy of the document below) and reported out their answers. After this quick review, my students were each assigned a specific solution. I had made a list of 10 solutions that repeat depending on how many students I have in class (I have 32 this year). The activity is easier when there are multiple students with the same solution. It makes finding a “date” faster. I made the quantities easy enough that students can quickly calculate and compare because the objective of the lesson was to recall the method to calculate the pH quickly, not to stump the students. Each student was asked to determine their individual pH to complete their personal “Solution Biography.” Then it was time to speed date! For each date, the students needed to find a specific match and determine their combined pH value.
For date #1, students were asked to find a student holding a match where both solutions were of the same strength and classification (weak acid matching another weak acid or strong base matching another strong base). They were asked to calculate their combined pH value by calculating their combined moles, combined volume, combined Molarity and finally combined pH. We reported out a summary of what happened to the pH (when two of the same concentrations were mixed, the pH remained the same but when two different concentrations were mixed, the pH changed).
For date #2, students were asked to find another student with a similar solution of a different strength (for example a weak acid had to find a strong acid). They had to realize that the stronger solution determined the pH, but only after being diluted by the weak solution.
Date #3 was tricky! The students had to find a partner to make a buffer solution with. Some students had a difficult time finding dates, so after the initial dates were found we had a pool of students in the center of the room to help everyone find a match. I went around and checked the solutions to confirm their match. The biggest take away was that the students realized they needed to pair up to achieve a solution in which the weak solution contains more moles than the strong solution.
Our last date required students to find a match to create a salt. They had to estimate the final pH, indicating whether the solution would result in a neutral, acidic, or basic salt. The students had to find a match with equivalent moles. After all students were matched, each pair read their individual solutions and in unison reported out their salt type and estimated pH (7, higher than 7, or lower than 7). The trickiest examples were matches in which both solutions contained weak electrolytes (in which case the students had to compare K values).
You can make modifications to the activity, but keep in mind when you are modifying the solutions list that the trickier the numbers are, the more difficult it will be for students to find a match quickly. It is important to make sure everyone will indeed have a match for each date. For uneven number of students, I had two struggling learners pair up as one solution. You may also choose to differentiate the students by assigning weak solutions to your best performers and strong solutions to your struggling learners. We completed the activity in 50 minutes. I observed smiling, excited students eager to find matches. After class I asked a few students what they thought of the activity. All five students said it was more fun than normal problem sets and they would love to play again. Another student remarked that she “understands dilutions and strength much better now.” And my favorite quote of the day was “I can’t leave my match, we made a perfect buffer and we resist change!”
acid base, neutralization, pH, concentration, molarity,
One class period
Pre-made sets of cards containing one of each of the following solutions. Repeat as necessary to accomodate for the number of students.
List of Solutions Used
10mL 1.0M HA Ka=2.0x10-7
10mL 1.0M HCl
10mL 1.0M NH3 Kb=1.8x10-5
10mL 1.0M NaOH
20mL 1.0M HA Ka=2.0x10-7
20mL 1.0M HCl
20mL 1.0M NH3 Kb=1.8x10-5
20mL 1.0M NaOH
40mL 1.0M HA Ka=2.0x10-7
40mL 1.0M NH3 Kb=1.8x10-5
Assign Calculating for pH review worksheet to be completed at the beginning of class.
Distribute one card per student identifying them as a specific acid or a base and their concentration. Students will complete a series of four "dates".
Create the cards. You can laminate them and reuse.
I created the activity for my students.