Mole Conversions: Moles to Mass (day 1)

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TSWBAT perform mathematical computations converting moles of a substance to mass.

Big Idea

Students must first learn how to do basic mole to mass and mass to mole conversions to understand stoichiometry.

Introduction and NGSS alignment

Performance Expectation (PE)/Disciplinary Core Idea (DCI)

This lesson is aligned with HS-PS1-7, the uses of mathematical representations to support the claim that atoms, and therefore mass, are conserved during a chemical reaction; and DCI-PS1.B, the fact that atoms are conserved, together with knowledge of the chemical properties of the elements involved, can be used to describe and predict chemical reactions.  However, students will need to perform mathematical computations to understand HS-PS1-7 which will require student to have a basic understanding of scientific notation, unit conversions and factor labeling (or proportions).  This lesson introduces students to the skill of converting moles to mass and mass to moles.  This skill will later help students understand stoichiometry and how the mass of one substance can be used to determine the mass of another using a balanced chemical equation.  

Science and Engineering Practices (SP)

HS-PS1-7 is one of the few high school Performance Expectations with the primary focuses on the use of mathematics to explain a concept.  Mathematical and computational thinking at the 9–12 level builds on K–8 and progresses using algebraic thinking and analysis.  Using computational thinking, students will build on the knowledge of the mole and molar mass to convert from one unit to another.

Crosscutting Concept (CCC)

One Crosscutting Concept that shares a close tie with PE HS-PS1-7 and SP, Mathematical and computational thinking is Scale, proportion and quantity.  In this lesson students use a proportion to navigate from moles to mass, in order to determine the quantity a substance.  It's important that a high school student understands that one unit of measurement can be proportionate to another.  Making this transition can help students have a better understanding of chemistry and science as a whole.  The bigger picture of Scale, proportion and quantity can be seen in a dynamic video from Bozeman Science on The NGSS standards, Scientific Practices and Crosscutting Concepts.  I highly suggest watching the videos if you are looking for a greater understanding how to incorporate NGSS into your lessons.

Debrief/Exit Slip

10 minutes

The previous day students learned how to calculate molar mass.  At the beginning of class students are asked to take out the previous day’s assignment so I can check it in.  My standard check-in for homework is 10 pts for full completion, 5 pts for half completion and zero points for less than half completed. Since the previous lesson gave student 20 minutes to complete the assignment, most students score a full 10 points.  The check-in takes about 3 minutes to complete.  Any student that received less than full credit will have time to complete the assignment before the end of the period.

After the check-in I ask if anyone has questions.  If so, we will take a few minutes to work out the problems.  When done with the Q/A, students will complete a will complete an exit slip to show their understanding of how to calculate molar mass.  On the overhead I have 3 molar mass problems ranging from simple to difficult.  I instruct them to answer the questions on a ½ sheet of notebook paper and show work.  I let them know that each problem is worth 3 points, one for work, one answer and one for units.

  1. NaCl  22.99 + 35.45 = 55.5 g/mol
  2. H2SO2(1.01) + 32.07 + 4(16.00) = 98.1 g/mol
  3. Ca(NO3)2 40.08 +2(14.01) + 6(16.00) = 106.1 g/mol

After 5 minutes I have them switch their paper with the person next to them, sign the bottom and grade the paper.  Next I solve the problems on the board letting them know that each problem is worth 3 points, one for work, one answer and one for units.  After grading they hand them back to look at their score and I collect them.  Students that could not solve any of the problems will be grouped together later in the lesson, so that I can work with them on molar mass before moving on to mole conversion problems. 

Later on in the lesson I check the exit slips for accuracy and who did not perform well on the assessment (scored a zero).  After reviewing the exit slip later in the lesson, only a about 3-4 students (15%) needed further assistance in calculating molar mass.  These students got all three problems wrong and needed to be walked through the process with further guided instruction. There was a percentage (30%) of students that got problem 3 wrong, but any students that struggled with this problem only need more practice and could get this by solving the mole-->mass problems that will be assigned later in the period. Since molar mass is involved in the majority of the problems throughout the unit, most students will be able to learn this skill since they will have continuous practice.


20 minutes

While I am pulling up the PowerPoint I instruct the class to take out their notebooks so we can take the next set of notes that we started the previous day.  This part of the Power point has two parts to it:  One that teaches how to calculate moles to mass and one that shows mass to moles. 



These notes should take only 15 minutes before moving onto guided practice.  The main purpose of the Power point is to provide students with organized notes that students can use as an examples. While students are finishing up the last slide I hand out the guided practice assignment.   

Guided Practice

20 minutes

The best way for students to master molar conversions is to practice a variety of problems.  Since this is a new concept I like to do guided practice.  Guided practice provides students with the opportunity practice a couple of problems and receive immediate feedback.  Students are then able to correct any mistakes and adjust their thinking.

While students are copying down the last page of notes I handout the practice problemsThis is not a worksheet I created, I found it on the internet.  I want to give credit to the person who created this worksheet, it provides a variety of problems that give students a sufficient amount of practice calculating molar mass. 

My standard method for guided practice is to a problem as an example, have the class do the next problem and then go over the problem. After doing this a couple of times, I have students work at their own pace and post a few problems at a time so students can get immediate feedback.   While students are doing the problems I group any students that are struggling and need help together and work with them.  The students that struggled with molar mass (did not show mastery on the exit slip) will be grouped together so that I can work with them on calculating molar mass and molar conversions while monitoring their progress.  These students will probably not work at the same pace as the rest of the class, but will still work through the same problems to ensure they are keeping up with the new content.

These problems take a significant amount of time longer to do and require more math skills to complete.  After doing a couple of practice problems, I instruct individuals that they can work with a partner to complete the problems.  While students are working on the first guided practice problem, I have a chance to look at the exit slip to see who need to be grouped to receive help from me.  Once I finish with two guided practice problems, I allow students to work through the problems at their own pace.  After several minutes, I post the answers to a couple of problems so they can check their work. 

I remind students to use the mole highway when solving the problems, always show work and write down units and not to take short cuts.  Common short cuts students take are not writing down the chemical formula, not showing work and forgetting to use units (see Molar mass Student work).