A Target Molarity Lab

homemade hydrometer in a graduated cylinder with salt water

“What are we doing to help kids achieve?”

Haitti in suggests that one of the most effective ways to improve student learning is for teachers to work together. This collaboration within a group of teachers trying to help students is a powerful form of professional development. The (local to me) has supported a group of teachers that meets about once every few months. I am privileged and honored to be part of this group. It is from this group that I was able to get an excellent lab activity. This activity was developed by Aimee Hansen at Mason High School who adopted it from Bob Becker.

First, Aimee used about fifteen forty milliliter glass dropper bottles. She filed a mark at the top of each one. She then placed each one on a balance. By adding water to the mark and recording the mass of the water, Aimee was able to accurately convert the mass of water to a volume. She then had several homemade mini volumetric flasks. Each flask has a slightly different volume. 

Students began the unit by learning and doing molarity problems. Prior to the activity, those students said they could successfully solve molarity and dilution problems. Small groups of students were each provided with one of the custom made volumetric flasks with a unique volume different from any other group. First the groups had to calculate how much sodium chloride they would have to dissolve in their flask with water to get a 1.45 molar solution. Then, in part II, they had to calculate what volume of a 2.05 M stock solution of sodium chloride they would need to make a 1.45 M solution in their volumetric flask. Students were instructed that once they made their solutions, they would give them to the instructor.


Figure 1: Homemade hydrometer made using a disposable pipette bulb, bb's and red & blue permanent markers

 

I tried this with my own students. As I accepted a solution from a group, I poured it into a graduated cylinder. I then placed a homemade hydrometer (a plastic pipette bulb with some bb’s in the bottom) into the cylinder (see figure 1). The hydrometer was calibrated to a 1.45 M solution that I had prepared previously. If the hydrometer floated in the group's solution so the “blue” mark was level with the surface, the concentration was confirmed to be 1.45 M salt.  Every “red” mark above or below resulted in a reduction in points.

Students were a bit shocked. They quickly learned that it is one thing to say they can calculate a concentration or dilution. It is another task completely to take that number and actually have to make the solution. They were extremely careful in their measurements and transfers. Many students second guessed themselves and a few asked if they could do it over because they thought they dropped some salt or solvent. The hydrometer provided instant feedback for the students. 

Overall, this lab activity/practical in my mind is a keeper. It provides immediate feedback, is easy to grade and forces students to work on their analytical lab skills.  Student engagement was at an all time high. This lab truly shows the power of target labs. I never would have learned about this without the help of my friend, Aimee, and the local American Chemical Society group. If you are not already a part of a professional development group with teachers, try to start one. Or better yet, consider submitting your ideas to this forum. We are all better off when we work together to help students. Thanks Aimee and Bob.

Concepts: 
Collection: 

Safety

General Safety

For Laboratory Work: Please refer to the ACS .  

For Demonstrations: Please refer to the ACS Division of Chemical Education .

Other Safety resources

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

 

NGSS

Mathematical and computational thinking at the 9–12 level builds on K–8 and progresses to using algebraic thinking and analysis, a range of linear and nonlinear functions including trigonometric functions, exponentials and logarithms, and computational tools for statistical analysis to analyze, represent, and model data. Simple computational simulations are created and used based on mathematical models of basic assumptions. Use mathematical representations of phenomena to support claims.

Summary:

Mathematical and computational thinking at the 9–12 level builds on K–8 and progresses to using algebraic thinking and analysis, a range of linear and nonlinear functions including trigonometric functions, exponentials and logarithms, and computational tools for statistical analysis to analyze, represent, and model data. Simple computational simulations are created and used based on mathematical models of basic assumptions. Use mathematical representations of phenomena to support claims.

Assessment Boundary:
Clarification: