Chemistry PBL
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Level I
Essential Investigations
Investigation: Empirical Formula
Description
Students design and conduct a laboratory process to determine the empirical formula of magnesium oxide. The context of this learning experience is after students have a good understanding of chemical formulas, moles and percent composition. Students may compute percent error using experimental ratios and theoretical ratio of magnesium to oxygen.
Learning outcome
• Plan and implement a laboratory investigation using safe practices
• Apply concept of formulas, mole, percent composition and process skills to determine the empirical formula of magnesium oxide.
• Make measurements with accuracy and precision • Use significant digits in measurement and calculation
Lab Management
1. Collect lab equipment and materials. Set equipment on each table for each lab group or instruct students to pick up materials from a central location. 2. Provide samples of magnesium for each student group.
3. After completing investigation, ask students to clean and put away the equipment in the proper location.
Safety
Students will review the laboratory safety precautions based on their plan. They will also review MSDS of the chemicals used in the investigation and identify hazards and safe handling/disposal of the chemicals.
Attention to chemical laboratory safety is essential as students will be using glassware and heat. Instruct students to wear protective aprons and goggles throughout the investigation. Magnesium is combustible and care must be taken while handling it.
Materials for each student group
Crucible with lid Balance
Gas burner Crucible tongs Clay triangle
Ring stand with ring support
Pieces of exposed magnesium ribbon Calculator
MSDS of magnesium
MSDS of magnesium oxide)
Instructional Process
Design
1. Present the challenge – to determine the empirical formula of magnesium oxide using
the given piece of magnesium.
2. Allow students to discuss and come up with a plan.
3. If students are unable to come up with a plan, some prompting questions may help with the design/problem solving process. Some suggested questions are – • What is the known empirical formula for magnesium oxide?
MgO
• What is the difference between empirical and molecular formula?
Empirical formula is reduced to lowest ratios. Example – CH
Molecular formula gives the exact number of atoms in the molecule. Example – C6H6
• When magnesium is burned in air, magnesium combines with oxygen from the air to form magnesium oxide. This may be stated as:
Mass of magnesium + Mass of oxygen = Mass of magnesium oxide. According to which law is this statement true?
The mass of reactants is equal to the mass of products. The Law of Conservation of Mass states that the total mass remains constant during physical or chemical change.
• Among the 3 masses described in the chemical reaction, which can be measured?
The mass of magnesium can be measured first. After the reaction, the mass of magnesium oxide can be measured. The mass of reacting oxygen cannot be measured directly.
• Will the mass of the magnesium oxide be greater or lesser than the magnesium used in the reaction? How do you know?
The mass of the magnesium oxide is likely to be greater than the magnesium because magnesium oxide has oxygen in addition to magnesium in it.
• How can we use what we know to find the formula of magnesium oxide? (Allow think time before soliciting student responses.)
The massed magnesium piece can be heated to react with oxygen from air. Magnesium oxide will be formed and this can be massed. We can find the mass of oxygen by subtraction and calculate the mole ratio from the mass ratios.
4. Allow students enough time to develop and record their plan for investigation using the available materials.
5. Monitor students as they work but allow students to create their own plan.
6. Allow students to share their plan for investigation. Ask students to list and explain the use of laboratory equipment needed for the laboratory process. Ensure that students follow correct terminology.
7. Instruct students to review the MSDS for the chemicals used and discuss appropriate
8. Instruct students to review the plan for investigation and identify safety precautions that will be necessary during the investigation. Allow time for students to record the laboratory and personal safety measures to be followed.
9. Model and discuss correct laboratory procedure for this activity. A brief procedure or
task list is shown - Procedure:
• Ensure personal safety by wearing aprons and goggles.
• Find the mass of clean, dry, empty crucible and lid. • Coil the piece of magnesium ribbon loosely and place it
at the bottom of the crucible.
• Determine the mass of lid, and crucible with the magnesium inside it.
• Place the crucible without the lid on the clay triangle and heat strongly till the magnesium ignites. DO NOT LOOK DIRECTLY AT THE BURNING MAGNESIUM! When the magnesium lights up, turn off the burner. Immediately place the lid on the crucible using tongs. Make sure to leave a small gap so the magnesium ribbon can react completely.
• After the apparatus cools to room temperature, determine the mass of crucible, lid, and the magnesium oxide inside it.
10. Ask the question, “How can we ensure accuracy and precision in the experiment?”
After students have responded, tell students that due to limitations of time and
materials only one trial will be conducted so accuracy of procedure and measurement is very important.
Investigate
11. Ask students to perform the investigation, record data, then discuss and write down calculations and conclusions. Monitor students and the learning process. 12. Allow student groups enough time to complete their investigations and analyze their
results and analysis. Encourage small group discussions and collaborations. 13. In the applications and extensions stage students may need time and resources to
research areas of their interest. In this investigation, students may examine commercial use of chemical formulas, mole ratios, etc. (Example: Which fluoride toothpaste has the greatest amount of fluoride?)
Report
14. Allow class time for student groups to share the results of their investigation, analyses, extensions, and applications. Peer student groups may be coached to assess the presentations.
15. Student work may be displayed physically or digitally for other students’ review.
16. At closure, students should demonstrate understanding of applications of chemical formulas in scientific research, industry and in the real world. This may be facilitated through questioning and discussion. Students should find percent error using
experimental and theoretical values. Students should discuss and find a way to compare theoretical and experimental values. Use of ratios or molar mass to find error is within the scope of the course. Sample scaffolding questions and student responses shown below may be used as a guide. However, class data and responses will vary.
• What is the theoretical mole ratio of Mg:O in magnesium oxide?
Mg:O = 1:1
• What is the experimental mole ratio? (use significant figures)
Mg:O = 1.20 : 1.00
• How can we use this information to calculate percent error (or accuracy)?
The empirical formula determined experimentally was accurate but did have some error since the mol ratio of 1.2 was rounded to 1.
Option 1
The percent error can be calculated from molar mass –
Theoretical Molar mass for MgO = 40.304 g/mol Experimental Molar mass for Mg1.2O1 = 45.165 g/mol
12% Error
% Error = (45.165 – 40.304 / 40.304) * 100 =
Option 2
The percent error can be calculated from mole ratio – Mg/O = 1.00 (theoretical) and Mg/O = 1.20 (experimental) (1.20 – 1.00) / 1.00 * 100 = 20% error
Students may give a different % error for the same set of data as shown above. This can lead to discussions about improper use of experimental data for promotion of commercial products.
• What are some of the sources of error?
Student Investigation Template
Title:
Question
Write the question for investigation.
Design/Plan
Investigation
Identify the purpose of your investigation and record it in the space below. Conduct your investigation. In the space provided, record - materials, experimental procedure, safety precautions, data, calculations, graph (if applicable), analysis, conclusions, and
application/extension of learning. Be prepared to discuss your procedure and results with the group.
Purpose
Materials
Procedure: (if different from your original design)
Safety Data
Analysis and Conclusion
