Mole Conversion: Moles to mass (day 2)
Lesson 9 of 9
Objective: TSWBAT perform mathematical computations converting moles of a substance to mass.
The school I teach at has a resource day once a week where classes are 42 minutes instead of 50 minutes. On resource days I try to plan lessons that have quizzes, test or summmative type assessments. I find that a 42 minute period is not long enough to do most lessons and provide students with a formative assessment. Today my students will perform a practice test to help them prepare for the test.
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. 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 reinforce the skill of converting moles to mass and mass to moles, in addition to helping students develop a deeper conceptual understanding of the mole. 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.
This is the third day students are working on their conceptual understanding of the mole as a counting unit and molar conversions. The idea behind this lesson is that students will develop a more confidence when solving molar conversion and understand how the mole is important in chemistry.
As students walk into the class they are instructed to take out the previous day’s assignment for check-in. This is the third day in a row I have checked-in the previous day’s assignment, so students are very familiar with the routine. The assignment is grade as 10 pts for completion, 5 pts for half completed and zero points for less than half completed.
While I am checking-in the assignment, students will watch How Big is a Mole? on EdTV. This video provides another explanation how the mole is used as a counting unit in chemistry. It is very similar to the BrainPOP (The Mole) and will provide a review that scientist use the mole to explain how many particles are in a sample of something. After watching the video we have a think pair share with each group providing a one sentence summary on a white board. Some examples of one sentence summaries:
1. The mole is a very large number that scientist use to count molecules, it’s 6.02 x 10^23.
2. The mole is not a furry creature, it’s named after a scientist that realized there is a large number of atoms in one a mole sample of a substance.
3. Just like a dozen tells us a quantity of doughnuts, the mole can tell us the quantity of particles in a sample of chemicals.
Having students participate in a think pair share with this video makes them discuss with another student how they perceive the mole and share it with the class. After each pair completes the white board they share it with the class. This takes about 5 minutes.
This part of the lesson is practice Quiz 6.2 molar mass_mass to mols that tests student’s conceptual understanding of the mole and the ability to perform molar conversions from moles to mass. I do not let students know this is a practice quiz until after they complete it, I believe it builds confidence for students that know the material and a sense of urgency for the students that need more practice.
After the warm-up I ask students if they have any questions about the previous day’s homework. If there are any question I will go over them to give students one last review before the pop (practice) quiz. After the review tell students to take out their calculators and periodic table and handout the practice quiz. They have 30 minutes to complete the quiz. Thirty minutes is enough time for the students that understand the material to complete the quiz. After they complete the quiz they can turn in and it will be graded and returned the following day.
The quiz consist of 12 problems and one extra credit question (see Quiz 6.2 key). The first four problems are molar mass problems and are worth two points apiece, problems 5 – 8 are molar conversions problems worth 3 pts (1 for work, 1 for answer and one pt for units) and problems 9-12 are true/false worth 1 pt. I believe this to be a sufficient quantity of questions to quiz students on the concepts they need to know. They start off easy and get progressively more difficult.
Since I do not want to move on to any new material, I do not have a post-quiz assignment. While students are taking the quiz I find this to be a good time to hand out grade sheets so that they can see how they are doing in the class. I also let them know that this would be a good time to make note of any missing assignments, get them completed and turned in over the next day or two.
My general policy is to get quizzes graded and turned back to the students by the following day. I find that students like this because they get timely feedback and can assess where they need improvement. It also gives me feedback on which students need additional help.
For this particular quiz, I am allowing students an opportunity to make quiz corrections to receive full credit for the quiz. Even though this is a practice quiz, I still provide students with points for completing it, just not quiz points. I make the the quiz worth 24 points, so if students complete the quiz corrections they will receive 24 homework points (15% of total grade) instead of 24 quiz points (20% of total grade). I still provide them with points because I want students to value the practice quiz but not to receive inflated grade points for completing quiz corrections. Basically I treat this quiz as an in-class assignment that all students will complete with 100% accuracy.
Overall most students completed the calculation (molar conversion) portion of the quiz successfully. The area where most students struggled was understanding terminology and the true false portion of the quiz, particularly question 11 and 9. Many students struggled with problem 11, trying to understand that that mass of one mole of O2 is 32 grams and therefore two moles would be 64 grams. I wasn’t too concerned with this as long as students got the computation problems (5-8) correct because this shows me that students could think through a problem mathematically and possibly just struggled with seeing a molar conversion problem in a different context.
The second area where students struggled was providing complete and accurate units for problems. There was a significant number of students that would get points deducted because they would calculate the molar mass and solve the problem correctly, but would not provide the correct unit (or any unit at all). I was stickler for this and would deduct 1 point per problem even if they got the problem correct. In my opinion these are easily obtained points if students have thoroughly practiced the problems, plus they are necessary if students are to truly complete the problems correctly and illustrate understanding.
Overall the objective of this lesson was met. By the end of this lesson, which carried over into the next class period with quiz corrections, student were able to assess their understanding of molar conversions and determine if more practice was necessary before moving on to new material. The quiz corrections provided students with one more opportunity to learn how to complete the problems with 100% accuracy.