In this experiment, students are asked to collect data in order to prepare a calibration curve showing the relationship between the % of salt and the mass of salt water. Using this graph, students will be able to determine the concentration of salt in a sample of ocean water. The activity provides the opportunity to practice and apply graphing skills.
Before the Activity
1. Teachers will have to prepare the salt solutions in advance or assign this to students as a pre-lab expercise.
The 1% solution is prepared by combining 1.0 grams of salt with 99.0 g of water. 2% salt water is a combination of 2.0 g of salt with 98.0 g of water. 3%, 4%, and 5% are prepared in a similar fashion. Each student group will need approximately 50 mL of each solution.
2. Electronic balances, sensitive to 0.1 gram, are recommended for speedy collection of data.
3. Ocean water is approximately 3.5% salt. If ocean water is not available, you may prepare a substitute by combining 3.5 g of salt for every 96.5 g of water.
4. Using plastic syringes makes it easy to quickly measure out 20.0 mL samples of water. (I obtained a class set of syringes from my veterinarian.)Have students practice drawing up 20.0 mL samples of water until they can do so without getting air bubbles in the syringe.
During the Activity
Provide students with a copy of the lab directions.
Students determine the mass of 20.0 mL of 1, 2,3,4 and 5% salt water solutions. Students make a calibration curve by graphing the mass of each solution versus % salt. A 20.0 mL sample of ocean water is then massed. Using the calibration curve, the % of salt in the ocean is found for the sample of ocean water by tracing along the graph.
After the Activity
After the lab, student will print out the calibration curve and answer the post-lab questions.