Acid and Base Calculations

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Objective

Students will be able to calculate the pH, pOH, [H+], and [OH-] for solutions as demonstrated by taking notes, performing practice questions, and completing an activity.

Big Idea

Acids and Bases have characteristic pH, pOH, [H+], and [OH-] values which can be calculated.

Introduction

In this lesson students are continuing to learn about Acids and Bases in terms of how to express these types of solutions quantitatively.  They do this through taking notes, making a chart, and performing a practice question with a partner. 

  • This lesson does not align with any specific performance expectations with the Next Generation Science Standards; however, I feel that it is important that students understand how to characterize acidic/basic solutions and be able to perform pH calculations before they leave high school.  This is important for two reasons.  First, for students who go onto college level chemistry they will be expected to have already seen these concepts.  Secondly, there are many everyday issues related to acids such as acid rain, ocean acidification, and medications.
  • This lesson aligns with the Next Generation Science and Engineering Practice 5: Using Mathematics and Computational Thinking.  It does so because students are learning to use pH calculations to characterize solutions, particularly how pH is on a logarithmic scale.
  • This lesson aligns with the Next Generation Crosscutting Concept 3: Scale, Proportion, and Quantity.  It does so because students are learning about the relationship between pH and H+ in terms of a logarithmic scale.

For this lesson there are no additional resources needed. 

Explain

40 minutes

I begin this lesson by introducing students to the concept of pH, the pH scale and calculations involving pH through a PowerPoint.  Students are a little familiar with the material because they were introduced to the pH scale in the previous lesson, Introduction to Acids and Bases, with station 7 in the introduction to Acids and Bases Lab.  

I open up the unit8 lecture 5 PowerPoint and pass out the unit8 lecture 5 student notes graphic organizer.  

  • I begin with slide 2 and go over the pH scale making sure to tell students that they need to memorize this scale.
  • I then go onto slide 3 and go over the equations to relate pH and H+.   I show them the visual relationship between H+ and pH so they can see how the equations are related, but do not go into lots of details because we will explore that more in the elaborate activity.
  • On slides 4 and 5 I have students practice using these equations.  I go through each example one at a time.  I show the question and go over the problem solving strategy that we have been using all year.  This includes underlining what students know, circling what they want, and figuring out how to get there.   Also, as students are doing this I make sure to show them how to plug these into their calculators. For more details on this see my reflection on Calculator use in chemistry.
  • On slide 6 I show students how pH and H+ are related in terms of the change in acidity of a solution, and then on slide 7 have them practice what they learned.  
  • On slide 8 I introduce pOH with the equations that relate pOH with OH- and pOH with pH.  I chose to not teach students the H+ related to OH- equation this year because of time as that students often struggle with the math on that equation.
  • On slide 9 I summarize the four scales and show students how they are visually related.  This visual is very helpful for some students but I tell them that we will discuss it in more detail by making a chart in a few minutes.
  • On slide 10 and 11 students practice using the new equations they learned and then on slide 12 I summarize how H+ and OH- are related in neutral, acidic, and basic solutions.

Elaborate

30 minutes

To help reinforce the pH scales and give students practice with using the equations I have them perform a Practice Paper.   

I start by passing the paper out to students and then leading them through doing the front section.

I have them start with Part I and I model how to fill in the chart.  I tell them that they do not have to write the standard notation for H+ and OH- but that they should notice the ones already done to get a visual of the quantities.  I do the first few rows with them and then have them complete on their own.  

As students finish Part I, I lead them with Part II.  I encourage them to use markers to label the different parts of the chart. I have them start with neutral, then do the acids, and finally finish with the bases.  

With Part III I try to get students to see how the equations that relate these four quantities make sense.  Students have a fairly easy time with the first one, but find the other three more difficult. This movie shows me helping students with seeing the relationships between the quantities in equation form. 

After reviewing the equations I have students do part IV on their own.  I encourage them to use their calculators as well as the chart on the front page to help them.  I also tell students that this is a great time to make sure that they understand how to plug these values into their calculators and to ask me for help if they are having problems. 

Evaluate

10 minutes

As a way to check that students are understanding how to use their calculators and how to plug and chug with the new equations I finish the lesson by having students do a practice question with their partner.  

I give each set of partners a question from the unit 8 group practice paper .  On the paper each group has a unique value for H+ and from that have them determine the pH, pOH, OH-, and if the solution is acidic or basic.

  • I like finishing with this activity because it gives me a chance to make sure that all students are really understanding how to solve these problems.   
  • As students finish the problems with their partners I walk around and check their answers. 
  • The biggest confusion that students have is determining the OH-, so as I walk around I tell them to use the pOH value that they calculated.