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# Gram to gram conversions

Lesson 5 of 8

## Objective: Students will be able to calculate the number of grams of reactants or products based on the number of grams that are given in a stoichiometry problem.

In this lesson students will be able to calculate the number of grams produced or needed in a chemical reaction using a balanced chemical equation. They start by reviewing mole ratio homework from a previous lesson. They then take notes on how to calculate the number of grams of reactants or products based on the number of grams that are given in a stoichiometry problem. Students spend the bulk of class time practicing these gram-to-gram conversions.

This lesson aligns to the NGSS Disciplinary Core Idea of *HS-PS1-7: Use mathematical representations to support the claim that atoms, and therefore mass, are conserved during a chemical reaction* because in this lesson students translate the relationships represented in balanced chemical equations to the macroscopic scale using the mole as the conversion from the atomic to the macroscopic scale.

It aligns to the NGSS Practice of the Scientist of *Using Mathematics and Computational Thinking* because stoichiometry requires students to use basic algebraic thinking and analysis.

It aligns to the NGSS Crosscutting Concept of *Stability and Change* because chemical reactions are about constructing explanations of how things change.

In terms of prior knowledge or skills, students should have a solid understanding of how to balance chemical equations and use mole ratios.

There are no special materials needed for this lesson outside of what I provide.

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#### Do Now/Activator

*10 min*

**Do Now**: Students start class by receiving the Stoichiometry Notes. I ask them to do as many steps of the procedure as they can. I give them a hint: the “limiting reactant” is CO_{2}.

I reason that this is a good way to start class because by now students should know how to balance chemical equations. Many also know how to convert from grams to moles and to use mole conversions. It would be very cool if a student can figure out how to do this solely based on the directions.

**Activator**: I then ask for a fist to four. Students who have completed all the steps should hold up 4 fingers, while students who have completed no steps should hold up a fist. If a student completes all the steps, I invite them to show their work to the class.

I have chosen this approach because I want to give students the chance to run with the directions if they want to. I also want to provide challenges of varying levels of difficulty so that all students can meet with some success even before I begin my instruction.

#### Resources

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**Mini-lesson**: Conducting gram-to-gram conversions is a multistep process containing several skills. First, I discuss the point of stoichiometry. It is designed to help chemists figure out how much reactant they need to make a specified amount of product or to predict how much product they will make based on a specified amount of reactant.

I explain that in our problems we will have an excess of one reactant and a limited amount of one reactant, and that a limiting reactant is like hot dogs and buns. You can only serve a hot dog in a bun until one or the other runs out. In that scenario, the hot dogs and buns are the reactants, and the hot dog/bun combo is the product. I note that my honors class has the added challenge of figuring out which reactant runs out first.

I then give an overview of the process as shown in this stoichiometry teaching video. Students check their work if they tried to do the whole problem, and they take notes or make corrections so that at the end of the mini-lesson they have a complete problem worked. Students have already performed each of the skills required, so my lesson is not designed to teach the individual skills, but rather, to show how the skills fit together.

**Guided Practice**: I then ask students to conduct the first practice problem in the stoichiometry practice problems. I circulate around the room to determine how students are doing. If they are proceeding without too much difficulty I wait until most people have worked through the problem, and then I ask a student to show his or her work. If the class as a whole is struggling, I work the problem with them, thinking out loud throughout.

This strategy will help me to release students when they are ready. Once most students are comfortable with the process, I can turn my attention to individuals who need more help. However, until that time, it is a smarter use of my time to provide leadership and guidance to the whole class.

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#### Application

*25 min*

**Student Activity**: Once most students have successfully completed a practice problem, I encourage them to focus on what they are doing when completing each step of each problem. I ask that they verbalize and record what they are doing because if they can articulate what they are doing, they will become better faster at doing stoichiometry problems. I tell them that I recognize that this is much more tedious than just doing the problems, and that is why I only want them to have 4 problems completed by the next class. To record their work, I ask students to use the Stoichiometry Articualtion Catcher.

I want students the work in this way because I want to train them to be mindful of the details. This strategy also helps because I can find solutions to student challenges with this work. For some students, they will be struggling because they do not understand the work, and for some difficulty will arise because they have not put enough though into understanding how the pieces fit together. By forcing students to slow down, they reveal their thoughts and this allows me to better meet the individual where they are.

**Catch and Release Opportunities**: Interestingly, we have covered all of the skills individually, but now students have to put them all together. These include balancing chemical equations, converting between moles and grams, and using mole ratios. Combining all of these skills presents a teaching challenge. Different students will react to this challenge in different ways. Some students will need re-teaching for one or more of these sub-topics, and so I conduct mini-workshops that students can attend for as little or as long as they want.

#### Resources

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#### Debrief

*10 min*

To wrap this lesson up I ask a student to participate in a conversation with me in front of the class. I explain that I want the student to explain in their own words how to solve a stoichiometry problem. I place much more emphasis on the procedure than I do on the answer. I listen carefully to each step, and then if I notice that a key piece of information was left out, I ask the student about it. In this video, a student walks through her process for explaining how many grams of H_{2}O she can get from 14 grams of O_{2} when the balanced chemical equation is 2 H_{2} + O_{2} --> 2 H_{2}O. The student articulation video starts after she has already explained how to balance the chemical equation.

Ending class this way allows me to highlight for students how important it is to pay attention to the myriad details that are in these problems. I re-emphasize how important it is to articulate the process. If you cannot explain how to solve the problems, then it is likely that you do not understand how.

The student work shown here is a good example of how far some of my students were able to get with this unit. The students understands the process--every problem is set up correctly. attention to small details kept this work from being perfect. In questions 2 & 7 a molar mass was miscalculated. In question 3 the wrong coefficient was used in the mole ratio. However, it is clear from this student's work that she completely understands the process.

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- LESSON 1: Evaluating Chemical Expressions
- LESSON 2: Balancing Chemical Equations
- LESSON 3: Chemical Reactions are Real
- LESSON 4: Mole Ratios
- LESSON 5: Gram to gram conversions
- LESSON 6: Stoichiometry Experimental Design
- LESSON 7: Conducting the Experiment
- LESSON 8: Critiquing the Lab Report