Finding the Percent Salt in Sea Water

Everyone knows that the ocean is salty, but exactly how much salt is in ocean water?

In this activity, you will create a “calibration plot” that establishes the relationship between the % of salt and the mass of samples of prepared salt water.  You will then find the mass of a sample of seawater, and by tracing along the plot, determine the % of salt in the seawater.

• balance
• 25-mL syringe
• prepared solutions of 1%, 2%, 3%, 4%, and 5% salt water
• a sample of seawater
• TI-84 graphing calculator

1. Set up the calculator by pressing . Clear any equation from Y1= by pressing . Now, set up the graph options. Press . Press . Turn on plot 1. Select   .  Make sure that you are set up to plot points in L1 versus L2. Then quit by pressing .
2. Press .  Select 1:Edit, then . If there is data in L1 or L2, clear it.  Since you are working with 1, 2, 3, 4 and 5% salt-water solutions, enter the values 1, 2, 3, 4, and 5 into L1.
   
3. Fill a syringe to the 20.0 mL mark with the 1% salt water.  Using the balance, determine the mass of the syringe with the water.  Enter this value into L2. Discard the solution in the syringe.
   
4. Refill the syringe to the 20.0 mL mark with the 2% salt-water solution.  Determine it’s mass and enter the data point into L2.
   
5. Repeat, finding the mass of 20.0 mL of 3%, 4%, and 5% salt water.  Record the masses of each solution in L2. 

6. Press .  Arrow over to CALC. Select 4: LinReg and press  . At the flashing cursor, type L1 , L2 , then press .  Arrow over to Y-VARS. Press . Select 1:Y1.  Press . Press . The parameters for the best-fit line will be displayed.  Press . Write down the equation for the line in the standard form, y = mx + b.

Press . Arrow down to 9: ZoomStat and press . The line of best fit will be drawn over your data. Press the  key and use the arrow keys  and  ro move along the line of best fit.

7. Draw up 20.0 mL of seawater into the syringe. Find its mass. Using the arrow keys, trace along the line until the y-value displayed at the bottom of the calculator screen matches the mass just measured.  Record the x-value at this point.
   
8. Tidy up the lab station.

Equation of line of best-fit __________________

 

Mass of syringe with 20.0 ml of seawater _________________
 
% of salt in sample _________________
       
Connect your calculator to the computer using a graph link. Copy and paste the graph and data table into a word document. Make a print out of the data and the graph for your lab report. Clearly indicate on the graph what the % of salt is in the ocean water.

If you need to save the data in order to print it out later, group L1 and L2 so that the data will be available to you later.

To Create a Group

1. Go to the memory key ( ).
2. Arrow down  to 8:Group. Press .
3. Select 1: Create New. Press .
4. Type in a name for the Group you wish to create. Press .
5. Arrow down to 4:List. Press .  Select L1. Mark it by pressing . Select L2. Mark it by pressing . 
6. Arrow over to DONE. Press .

When you are ready to retrieve and use your data, you will have to ungroup it.

To Ungroup

1. Go to the memory key  (). Arrow down  to 8:Group. Press .

 

2. Arrow over to UNGROUP. Arrow down to find the name of your group. Press . Select “Overwrite All”.   Press .  You will now find that your data is available  in L1 and L2. You can access it by pressing  and then .

1. Explain what a calibration plot is and how it is used.
   
   
2. State what you found for the % salt in seawater.  If the theoretical percentage of salt at room temperature is 3.2%, what is the percent error?
   
   
3. Think about and list possible sources of error.  How could the experiment be improved?