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Concentration and Solutions Objectives 1. Understand and create a linear calibration curve based on hydrometer readings 2. Calculate the ${w/v) concentration of a given salt solution using two methods. 3. Determine molarity of an unknown solution using calibration curves Background A hydrometer is a device used in winemaking and other practical settings where a quick and reliable determination of solution concentration is helpful. The principle behind this is that when the concentration of a solution changes, the density of the solution often changes with it In this tab, we will be studying salt solutions, which become more dense as the concentration increases. You may have heard that swimming in very salty water makes you float higher than in freshwater. A hydrometer is a floating instrument which is calibrated to measure this effect. Part 1. Linear Calibration of a Hydrometer 1. Watch the first video which shows the operation of a hydrometer in different salt solutions Notice that it is mostly filled with air, but also has a lead weight in the bottom to keep it floating upright. 2. Record the concentration and hydrometer scale reading for each solution. Study the scale carefully, and find where it meets the bottom of the meniscus 3. Using Excel or another program such as spenoffice.org you are very comfortable with Create graph showing hydrometer reading ly-axis) versus % NaCl (w/4) concentration de axis). Detailed instructions for this are in your la manual on pages 72-73. In short, you should enter the concentrations in column A and your hydrometer readings in column Insert an XY scatter plot (never a line graph. Add a trendline, including an equation in the form ymx+b, and label your awes. You should probably show the result to your instructor Plot the data and graph together on a single page, or attach the Excel file to a pdf file of your lab report 4. Write the equation for your line here: 5. What do the x and y variables in your equation represent? Xis _andy is 1 Part 2. Making a salt solution 6. Watch the second vides which shows the preparation of a salt solution with a precise concentration using a volumetric flask 7. In your notebook, record the mass of salt used to make the solution, and also the volume of the flask & Calculate the expected concentration of this solution in w/V) and in mol/L Show your calculations here:
8. Calculate the expected concentration of this solution in %(w/v) and in mol/L. Show your calculations here: 9. Record the hydrometer reading for the solution from the end of the video. Then calculate the % (w/v) concentration of the solution using the equation for the line on your graph you put down above. You are using a linear calibration! This is a common practice in chemistry. 10. Calculate the % error for the concentration you found using the hydrometer data. The theoretical value is based on the concentration you calculated from the mass of salt. 11. Can you think of any possible sources of error? List several here. When you are done, turn in this completed handout including your graph from Excel.