Experiment: Solutions Preparation, Part 1

1. Experiment: Solutions Preparation, Part 1 CHE116 Fall 2012

2. Purpose of This Experiment • To learn how to prepare solutions, including the lab technique of quantitative transfer • To become familiar with calibration curves and spectrophotometers • Work in groups of 2 CHE116 Fall 2012

3. Prepare A Solution Quantitatively transfer solute and dissolve solute with solvent Dilute to calibration mark on neck of volumetric flask Weigh solute CHE116 Fall 2012

4. Molarity Solutions The directions on the next 4 slides are specific for molarity solutions that are prepared with a volumetric flask. CHE116 Fall 2012

5. Weigh Solute Molarity = moles of solute / Liter of solution • You need to know the molarity you need • You need to know the size of the volumetric flask that will be used (the liters) • You need to calculate the number of moles of solute needed • You need to convert moles to grams CHE116 Fall 2012

6. Quantitatively Transfer Solute • Make sure you know exactly how much solute is transferred to the receiving vessel: • Rinse the residual solid on weighing paper into the receiving vessel • Reweigh the weighing paper to determine mass of residual solid and subtract this mass from the solute mass CHE116 Fall 2012

7. Dissolve Solute with Solvent(for volumetric flask) The volumetric flask is used for molarity solutions; it should either be completely dry or have some solvent in it (at least enough to completely cover the bottom) • Add solvent (deionized H2O) until the volume of liquid is a little below the neck of the flask • Swirl flask until all of the solid is dissolved (make sure you do not lose any liquid) CHE116 Fall 2012

8. Dilute to Calibration Mark Each volumetric flask has one calibration mark on the narrow neck. The position of this mark is unique to each flask. • Use a small beaker to pour water into the flask to bring the level of liquid into the neck, but still below the calibration mark. • Use a squirt bottle to carefully bring the liquid to the calibration mark. The bottom of the meniscus should be exactly on the calibration mark. • Seal the flask and invert 50 times to completely mix. CHE116 Fall 2012

9. % Weight Solutions • The directions on the next 2 slides are specific for % weight solutions. CHE116 Fall 2012

10. Weigh Solute % weight = [(mass solute) / (mass solution)]x100 • You need to know the %weight you need • You need to know how much solution you need • You need to calculate the mass of the solution (assume a density of 1.0 g/mL) • You need to calculate the mass of the solute CHE116 Fall 2012

11. Dissolve Solute with Solvent(for % weight solution) % weight solutions can be prepare in a beaker or Erlenmeyer flask. The Erlenmeyer flask is better because of the sloped sides and small opening. • Find the mass of the empty flask. • Find the mass of the flask and solute. • Find the mass of the flask, solute, and solvent. • Calculate the mass of the solution mass solution = (mass flask, solute and solvent – mass empty flask) CHE116 Fall 2012

12. Find the Absorbance and Concentration of Your Solution • Use a disposable transfer pipet (plastic) to almost fill a small test tube with your solution • Wipe the outside of the test tube (remove droplets and fingerprints) • Place this test tube in the sample compartment of the spectrophotometer (use the same spectrophotometer each time) • Record the absorbance readout from the spectrophotometer • Use the calibration curve provided and determine the concentration of your solution CHE116 Fall 2012

13. Spectrophotometer An instrument that will indicate the amount of light (of one wavelength) absorbed by a sample slit readout LED readout Light source monochromator detector sample CHE116 Fall 2012

14. Beer – Lambert Law A = abc A is absorbance of light by the sample a is the molar extinction coefficient b is the solution path length c is the concentration of solute This is a linear relationship; y = mx + b y is absorbance, x is concentration CHE116 Fall 2012

15. Calibration Curves • Calibration curves show the linear relationship between absorbance and concentration. Absorbance Concentration, M CHE116 Fall 2012

16. Calibration Curve • You will have an absorbance reading for your solution (from the spectrophotometer). • Find that absorbance reading on the y-axis of the calibration curve. • At that point on the y-axis, move horizontally across to the calibration curve line (use a ruler) • When you reach the line, drop straight down to the x-axis • The value that you are at on the x-axis is the concentration of your solution CHE116 Fall 2012

17. Calibration Curve Absorbance Your absorbance reading Your concentration Concentration, M CHE116 Fall 2012

18. Standard Deviation A method for calculating the ± for the entire experiment. It is an indication of the precision of the experiment; a small standard deviation is best. s = ∑ (deviation from the mean)2 (Number of trials – 1) ∑ is the sum of Deviation from the mean is the individual result minus the mean (use the absolute value) CHE116 Fall 2012

19. Next Week • Library Field Trip • CHE116 students will go through a tutorial for database searches (approximately 30 minutes). • This will be either at the start or near the end of the lab class. Ask your instructor for details. CHE116 Fall 2012