Summary

In this activity, students will determine empirical formulas for samples of trace evidence collected from various crime scene scenarios. They will use the information to link a criminal to a particular crime scene.

Grade Level

High School

NGSS Alignment

This activity will help prepare your students to meet the performance expectations in the following standards:

• Scientific and Engineering Practices:
  • Using Mathematics and Computational Thinking
  • Analyzing and Interpreting Data

Objectives

By the end of this activity, students should be able to:

• Determine the empirical formula from percent composition or mass data.

Chemistry Topics

This activity supports students’ understanding of:

• Percent composition
• Empirical formula

Time

Teacher Preparation: 15 minutes

Lesson: 30 minutes

Materials

• Case Description Cards
• Suspect Cards
• Calculators
• Periodic Table

Safety

• No specific safety precautions need to be observed for this activity.

Teacher Notes

• This is intended to be used an individual activity but can be differentiated to partner work.
• There are 8 “Case Description Cards” provided which each require the empirical formula to be determined. Each case is provided with percent composition data (ID: A, black text) as well as with mass data (ID: B, dark red text). Teachers can decide to use one type of card or the other, or a combination of both. Students should identify the same empirical formula and suspect for each case, regardless of which set of data they use.
• 8 “Suspect Cards” are provided and are used by students to match the empirical formula that they determine to the suspect of a crime.
• Students are required to show work for each case and determine whodunit on the Student Handout. Savvy students may notice the fact that NaCl is the only compound that contains chlorine, and sodium bicarbonate is the only other one that includes sodium (as well as hydrogen, carbon, and oxygen) – if so, good for them for paying attention! But remind them that they still need to show their calculations for each case on the student handout!
• Students should have a background on calculating empirical formula from percent composition or mass values (depending on the card type chosen by the teacher) prior to completing this activity. Students should also be familiar with differentiating between empirical and molecular formulas.
• The case cards may be solved individually. I suggest printing a set of case cards for 4 students to share in order to allow the cards to be passed efficiently. They do not need to be solved in any particular order, so students can take whatever cards are available in any order until they have determined the formulas for all eight cases.
• To differentiate, one could assign the entire set to two students to share the load. The red text cards (ID: B) may be a little more difficult to solve, and could be assigned to advanced students.
• You could print on cardstock or colored paper and laminate the cards to ease material handling and allow for better re-use between classes.
• Print out suspect cards and place them on a large “Suspect Board” (magnets or tape to large blackboard or whiteboard or semi-permanent poster). This makes it more interactive, as students become quite competitive to see who’s the best “detective.”
• Alternatively, if you don’t want students getting out of their seats so much, you could print the suspect cards for the groups as was done for case cards. In this case, suspect cards should not be distributed until about halfway into the activity to allow for clarity of tasks. (Students should have calculated most of the formulae before they get the suspect cards.)
• The extension activity is optional, but it could be a good way to encourage students to have some fun in the form of writing a silly story (and you don’t have to tell them and ruin the fun, but this will also help them develop writing/literacy skills!). This is also a good way to keep the early finishers engaged.
• Answer Key: