Lesson 11 of 12
Objective: Students will determine the concentration of a base by conducting a titration.
In a previous lesson students learned how to use titration data to figure out the concentration of an acid or a base. In this lesson students conduct a titration in order to determine the concentration of NaOH that is used to neutralize a known concentration and volume of acetic acid.
This lesson aligns to the HS-PS1-7: Use mathematical representations to support the claim that atoms, and therefore mass, are conserved during a chemical reaction because students must use a balanced chemical equation in order to calculate the molarity correctly.
It aligns to the NGSS Practices of the Scientist of Planning and carrying out investigations because titrations are an investigation into the properties of an unknown chemical concentration. It also aligns to the Practice of Analyzing and interpreting data because students will obtain a volume from their titration that they will then have to plug into a molarity formula in order to ascertain the molarity of the base.
It aligns to the NGSS Crosscutting Concept of Energy and Matter: Flows, Cycles, and Conservation because a titration reaction is a good example of how the total amount of matter in the closed system—in this case the Erlenmeyer flask—is conserved.
In terms of prior knowledge or skills, students should be able to solve titration problems prior to doing this lab.
The materials needed for this lesson include the following:
- A burette
- A 100 ml Erlenmeyer flask
- NaOH of “unknown” molarity (I used 0.2M)
- 25 ml of 0.15M acetic acid
- A funnel
- Eye dropper or disposable pipette
- Safety goggles
Do Now: Students enter the class and are asked to do the first 3 steps of the titration problem from the Titration Lab Directions. I reason that this is a good place for students to begin because they will need this information in order to make sense out of the data that they collect in lab today.
Activator: After students have had some time to work on this, I ask them how it went. Students struggled with a number of issues. First, the acid is written differently than they are used to—acetic acid has the H written at the end, and that threw some students. Secondly, many students tried to solve the entire four-step process. However, when I asked them where they got the volume amount in Step 4, many of them realized that the only way they could get this was by doing the lab. the first three steps should look like the ones found in this student's lab notes.
Mini-lesson: I begin the mini-lesson by asking questions about safety. What do you do if you get acid or base on your hands? (tell teacher and rinse with cold water for 5 minutes) When you do you have to wear goggles? (As long as someone is working at my table with glass, or acid or base.) When is it ok to dump acid or base down the drain? (Never—unless it has been neutralized.)
I then discuss the titration. I first show, using my document camera, that “0” is at the top of the burette, reminding students that we are measuring how much base leaves the burette. I remind them of the meniscus, and note that reading a burette is like reading a graduated cylinder, except you read from the top down.
I then model releasing the base into the acid. At first there is on color change, but then as I swirl the flask the color stays for longer and longer periods of time. I then show what happens when too much base is added to the acid—a dark purple color ensues. I note that if that happens it is because too much base is added, and next time a slower drop rate is required. I note that ideally the pink color will stay for 30 seconds, and disappear. However, if a very light pink color remains, that is still good. I note that if the dark pink color occurs, then acid must be slowly added to the solution in order to neutralize the base.
I then zoom out to the big picture. I remind students that their goal is to figure out the molarity of the base. I note that during our practice problems students retrieved the ml of base from the problem itself, and plugged it into the molarity formula. However, this time students will have to get this number from their burette. I remind them that they are tasked with doing the lab 3 times, and also with comparing data with a partner. Finally, I note that everything I have gone over is captured in the procedure for the Titration Lab.
Student Activity: After I am sure that students understand the procedure, I turn them loose to conduct the lab. My primary focus during this time is on lab safety. I encourage patience and thoughtfulness with students, and if I see someone doing something unsafe, such as leaving glassware near the edge of the lab bench, I notify them immediately.
I am patient during this time. While many of the titrations occur without difficulties, some students do get confused in as they conduct the procedure. For example, one student had acid in both their burette and their flask. Another student forgot to put indicator in their flask, and this video called supporting the titration lab shows how I react.
As students gather their data, they record it in the Titration Lab sheet as shown in this student's lab notes.
One thing that I need to do a better job with next year is teaching significant figures. I notice that one student records to the 1 ml, while the other records to the 0.1 ml. the first student was taking advantage of the graduations between each ml marking, while the second student was inappropriately rounding.
Another problem I have with this data is its imprecision. I suspect that this is due to the fact that the students use color change as their measuring tool. The lab directions instruct students to " Slowly add base to your acid, while gently swirling the flask, until it turns pink for at least 30 seconds." I think these directions are too vague and open to interpretation. I ran out of time during this unit or I would have asked students to modify and/or articulate a procedure that everyone follows uniformly with the goal of producing more precise data.
To wrap this lesson up I complement the class on their attention to returning the lab the way they found it. I thank everyone for having a safe lab in which no one got hurt and nothing was broken.
I then review with them what I am looking for in the lab write-up. I note that this is a short writing assignment designed to report on the molarity each student calculated. I note that there is space for students to compare their data with a classmate’s, and to evaluate the data based on how similar it is.
I was pleased with the results of the lab. Many students got close to 0.2 M for the NaOH. Here is an exemplary student lab write-up and one that is a typical student lab write-up. In both cases, the students show an understanding of the titration process.