Converting Between Masses and Moles

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SWBAT use the molar mass and dimensional analysis to convert between the mass of a sample and the number of moles in a sample.

Big Idea

Calculating the number of moles of a sample can be a quick and straightforward process using the factor label method.


Since students learned dimensional analysis the day before, we move now to applying it to chemistry.

When we introduced molar mass, we stressed that it was the number of grams per one mole of substance.  At the time, students didn't understand that the complex unit is a conversion factor.  Today I show them how to use the molar mass of a substance to convert between mass and moles.

True mastery of HS-PS1-7 will find students doing mass to mass conversions within a chemical reaction to prove the Law of Conservation of Mass.  To get students there, they must know how to convert from mass to moles and then back.  This lesson also utilizes Science and Engineering Practice 5, Using mathematical and computational thinking.

Opener -- Checking in Work

10 minutes

At the beginning of class, I ask students to get out their dimensional analysis practice from yesterday.  I walk the room and check off students who have completed the work.  I then display the correct answers on the document camera, and take any student questions or concerns.  Students note that a few problems required more than one step, since the conversions crossed over the 1 meter mark.  I ask them to disregard those, and apologize for not deleting them from the paper.

I ask students what made this task hard.  They respond "Not being sure what the conversions were".  Since students are very weak at the metric system, they were uncertain exactly what they were converting, even though they set up the conversions effectively.

I ask what made the task easy, and some students respond "Using the chart and writing the units" . Other students disagree, and say that it confused them.  I ask those who said they were confused to bear with us, because we will be using the chart more in the coming days and weeks.

I then put a copy of the Mass and Moles sheet on the screen, and pass it out to the class.

Whole Group Instruction

10 minutes

The front side of the Moles and Mass worksheet we are using is from the Instructional Fair Chemistry worksheets.  I like this particular resource for two reasons.  First, the ten conversions use the same five chemicals.  This saves us time from calculating ten different molar masses.  The second reason is that the conversions are clustered, with the first five converting from grams to moles, and the second five from moles to grams.  This consistent repetition is nice to build confidence in students' abilities to properly set up and solve the problems.

To begin, I ask each table to split up the five chemicals, and for everyone to find a different molar mass.  Whoever finishes first is to find the fifth one.  This saves us time and focuses on the task at hand: using the molar mass to convert.

After about three minutes, we have all the molar masses, and I ask tables randomly what they got and add it to my paper on the document camera.  This lets groups check their work, and ensures we all have the masses correct before setting up conversions.  If a group gives the wrong molar mass, other groups are quick to point out discrepancies.

I demonstrate the first set-up and calculation with students, as in this screencast.

The key points I make is that the units must cancel diagonally, and the final units on top should match what you are being asked for, in this case, moles.

I ask students to try number 2, and then check in with them to see how it went.  If it went well, I move on to the bottom of the page as a whole class.  If students are struggling, we take time to finish the top and I circulate and help struggling students.

Now, we move to the bottom of the page.  I ask "What's different about the starting information here?"  Students point out that we are starting with moles.  So I have them walk me through how to set up the dimensional analysis.  We then do the math together.

I give them a chance to work the second problem, and then we go over it together again.

I release students to work the remaining six problems on this page, and the back.  I point out that the back is a mix of the two types, so it will be very important that they read the problems and put the correct starting units to figure out what type of problem it is.

Individual Practice

25 minutes

As students finish the Moles and Mass sheet, and work onto the back side, I circulate the room, answering questions, checking work, providing encouragement and feedback.

Students struggle for a few problems on the back, having to dissect the word problem to determine what is given and what is asked for.  I pause the class and break down the first problem on the overhead using highlighters, and then set up the dimensional analysis from the information on the problem.

Once students see how to differentiate the given (consisting of a number, a unit, and chemical) from the unknown (consisting of just a unit and chemical) they fly through the process.  I notice very few errors in calculating the molar masses.

This student did most of the work correctly, but has a couple errors, such as the dinitrogen tetraiodide problem where he didn't find the molar mass.

Today I am less positive about how the units are being left out, and begin to stress to students that it is CRUCIAL to write their units.  In particular, I look for instances where not writing units caused them to set the conversion upside down.

All students finish the front of the page, and about half finish the back side as well.  Those who don't finish are instructed to complete it for homework.


5 minutes

To close the class, I point out how good they have gotten at finding molar masses of chemicals.  I praise their work on that, and remind them that tomorrow will be our molar mass quiz.

Students freak out at first, and then I assure them that there is no dimensional analysis on this quiz, just finding the molar masses using the formulas and the periodic table.

Any students who have completed both sides can turn in their paper before leaving.  The rest take the paper home to complete over night.