**"What are we doing to help kids achieve?"**

A friend that I work with teaches chemistry in a school within a school. She was trying to find a lab to help students review at the end of the semester. After a little brainstorming together we decided to use an idea from Bob Worley. Last May, I wrote a blog about a titration lab. Bob had posted a comment and proposed an idea that really stuck in my head. He suggested that instead of doing a formal acid base titration, why not do it through mass instead of volume? See the video below. Apparantly, back in the day, scientists believed that once instrumentation improved, gravimetric titrations would provide better results. Since scales were not that great at the time they turned to using volumes for analysis. Using volumes works well, but why not give gravimetric analysis a try? Maybe we could also turn this into a great review of the semester content.

**The Set Up**

I started by standardizing a sodium hydroxide solution of about 0.15 M with KHP. I then used the standardized sodium hydroxide solution to standardize two solutions of HCl. The two acidic solutions were about 0.20 and 0.10 M. I made 250 mL of each solution. I estimated that students could run multiple trials of titrations with these solution and I would have enough for about the next 5 years of class. Next, I made the "stand" for the berel pipette. I bought some really cheap plastic cardboard that is used for yard and election signs. I made a rectangle with a small open area in the bottom middle. I cut two slits in the rectangle and took another approximately two inch by twenty four inch piece that I bent and placed each end in the slits. This created a tripod type of stand (see figure 1). Above the cut out in the rectangle I poked some holes for plactic zip ties. I was able to slide the pipette into the ties. I also ordered some inexpenive "Hoffman clamps" to place on the fat end of the pipette (see figure 2). These clamps allow a student to carefully dial in the drops. The end of the pipette was pulled tight and then cut off. This made the drops much smaller. The smaller ends and the "Hoffman clamps" allows for students to carefully dial in extremely small amounts of liquid.

**Figure 1 -** Gravimetric titration set up

**Figure 2 -** Hoffman clamp on pipette bulb.

**The Process**

Students were instructed to obtain a small vial and put in a few drops of phenolpthalein. They were to find the mass of the vial and its contents. They then had to fill the vial about one third full with one of the two acids and record the new mass. Students then obtained the base. They filled the pipette with the special tip with the basic solution. They placed the pipette in the apparatus, added the Hoffman clamp and carefully added base to the acid until they saw the first permanant change of color. They recorded the mass of the vial and repeated the procedure for a second trial. Watch the video below to see exactly how the lab is set up.

**The Analysis**

There are many ways teachers can direct students to analyze the data from this activity and use it to review the material covered over the entire semester. Here are some questions that a teacher could ask.

** Nomenclature** - What are the formulas for the reactants and products?

** Reactions** - Given the reactants, predict the products and write the correct balanced equation for reaction.

* Significant Figures - *Given the masses recorded for the empty and filled vial, what is the mass of fluid added with the correct number of significant figures?

** Density -** Given the density of the acid and base solutions their recorded masses, solve for the volume of each.

** Moles** - Find the molar mass of KHP. If you have the grams of KHP and the molar mass of KHP, how many moles of KHP were used when the teacher standardized the base?

* Molarity - *The molar ratio of KHP to base was 1:1. Given moles of KHP and now the moles of base along with the volume of base, what is the molarity of the base?

** Stoichiometry** - How many moles of acid were required to react with the base? What is the molarity of the acid? How many grams of the other product should form?

I am sure you could come up with more questions. This could be used as a review activity, formative assessment or possibly even a lab practical. There are many possibilities.

**Special Thanks** - I want to note again that the set up for this was inspired by Bob Worley.

## Safety

### General Safety

General Safety

Please refer to the ACS Guidelines for Chemical Laboratory Safety in Secondary Schools (2016). Some additional information on these guidelines can be found in a Pick at ChemEd X.

#### Safety resources

RAMP: Recognize hazards; Assess the risks of hazards; Minimize the risks of hazards; Prepare for emergencies

## NGSS

Matter and its Interactions help students formulate an answer to the question, “How can one explain the structure, properties, and interactions of matter?” The PS1 Disciplinary Core Idea from the NRC Framework is broken down into three subideas: the structure and properties of matter, chemical reactions, and nuclear processes. Students are expected to develop understanding of the substructure of atoms and to provide more mechanistic explanations of the properties of substances. Chemical reactions, including rates of reactions and energy changes, can be understood by students at this level in terms of the collisions of molecules and the rearrangements of atoms. Students are able to use the periodic table as a tool to explain and predict the properties of elements. Using this expanded knowledge of chemical reactions, students are able to explain important biological and geophysical phenomena. Phenomena involving nuclei are also important to understand, as they explain the formation and abundance of the elements, radioactivity, the release of energy from the sun and other stars, and the generation of nuclear power. Students are also able to apply an understanding of the process of optimization in engineering design to chemical reaction systems. The crosscutting concepts of patterns, energy and matter, and stability and change are called out as organizing concepts for these disciplinary core ideas. In the PS1 performance expectations, students are expected to demonstrate proficiency in developing and using models, planning and conducting investigations, using mathematical thinking, and constructing explanations and designing solutions; and to use these practices to demonstrate understanding of the core ideas.

*More information about this category of NGSS can be found at https://www.nextgenscience.org/dci-arrangement/hs-ps1-matter-and-its-interactions.

**"**Matter and its Interactions help students formulate an answer to the question, “How can one explain the structure, properties, and interactions of matter?” The PS1 Disciplinary Core Idea from the NRC Framework is broken down into three subideas: the structure and properties of matter, chemical reactions, and nuclear processes. Students are expected to develop understanding of the substructure of atoms and to provide more mechanistic explanations of the properties of substances. Chemical reactions, including rates of reactions and energy changes, can be understood by students at this level in terms of the collisions of molecules and the rearrangements of atoms. Students are able to use the periodic table as a tool to explain and predict the properties of elements. Using this expanded knowledge of chemical reactions, students are able to explain important biological and geophysical phenomena. Phenomena involving nuclei are also important to understand, as they explain the formation and abundance of the elements, radioactivity, the release of energy from the sun and other stars, and the generation of nuclear power. Students are also able to apply an understanding of the process of optimization in engineering design to chemical reaction systems. The crosscutting concepts of patterns, energy and matter, and stability and change are called out as organizing concepts for these disciplinary core ideas. In the PS1 performance expectations, students are expected to demonstrate proficiency in developing and using models, planning and conducting investigations, using mathematical thinking, and constructing explanations and designing solutions; and to use these practices to demonstrate understanding of the core ideas."

Students who demonstrate understanding can use mathematical representations to support the claim that atoms, and therefore mass, are conserved during a chemical reaction.

*More information about all DCI for HS-PS1 can be found at https://www.nextgenscience.org/dci-arrangement/hs-ps1-matter-and-its-interactions and further resources at https://www.nextgenscience.org.

Students who demonstrate understanding can use mathematical representations to support the claim that atoms, and therefore mass, are conserved during a chemical reaction.

Assessment does not include complex chemical reactions.

Emphasis is on using mathematical ideas to communicate the proportional relationships between masses of atoms in the reactants and the products, and the translation of these relationships to the macroscopic scale using the mole as the conversion from the atomic to the macroscopic scale. Emphasis is on assessing students’ use of mathematical thinking and not on memorization and rote application of problem - solving techniques.