Chemistry 151 Last Updated: Dec Lab 2: Measurements & Chemical Changes

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1 Chemistry 151 Last Updated: Dec Lab 2: Measurements & Chemical Changes Introduction This lab will serve as an introduction to some of the basic techniques and skills you ll be using in this course (and most of your future science courses). It will also introduce you to the idea of measuring and recording in the correct number of significant figures, which were discussed in Chapter 1. In lecture, you re usually presented with data measured or recorded by someone else (usually in the form of a word problem) and you have to determine how many significant figures are present in each value. During lab, however, you are the one making the measurements, so it s your responsibility to record them in the correct number of significant figures. The number of significant figures will be determined by the instrument you use to make a given measurement. For example, a balance that can measure out to milligrams (0.001 g) would allow you to record masses with more accuracy and therefore more significant figures than one that only goes out to the first decimal place (0.1 g). We will also explore the concept of physical and chemical changes, which was also discussed in Chapter 1. You re not expected to know the details of any chemical reactions you might observe during this week s lab. That ll happen later in the semester. Procedure PART I: Accuracy & Precision A. Pipettes vs. Graduated Cylinders 1. Use a volumetric pipette (the good ones, not the smaller, disposable ones) to measure 10 ml of water. The fill line will be a small line above the fat section of the pipette. You may use a rubber bulb or plastic pump (see me if you re unsure how to use either). 2. Transfer the water to a 25 ml graduated cylinder. 3. Measure a second 10 ml of water with your volumetric pipette and transfer it to the graduated cylinder, adding it to your initial measurement (giving you a total of 20 ml). 4. Repeat steps 1-3 twice, for a total of three trials (empty your graduated cylinder between each trial). 5. Repeat steps 1-4 using 20 ml water and a 100 ml graduated cylinder. B. Graduated Cylinders vs. Beakers 1. Use a 25 or 50 ml graduated cylinder to transfer 50 ml to a 250 ml beaker. 2. Transfer another 50 ml (giving you a total of 100 ml) 3. Repeat step 2 again (giving you a total of 150 ml and ) then once more (for a total of 200 ml) 4. Repeat steps 1-3 twice, for a total of three trials (empty your beaker between each trial). 5. Repeat steps 1-3, this time using a 100 ml graduated cylinder to transfer a total of 400 ml to a 600 ml beaker. PART II: Combining Volumes A. Combining liquids 1. Using a volumetric pipette, measure 20 ml of water and transfer it to a 50 ml graduated cylinder. 2. Measure 20 ml of isopropanol (2-propanol) with a volumetric pipette and add it to the graduated cylinder. Gently swirl to mix. 3. Repeat steps 1 and 2 for a second trial.

2 B. Combining a solid with a liquids 1. Measure approximately 5 g of sodium chloride and add it to beaker ( ml). 2. Measure 20 ml of water using a graduated cylinder and add it to the beaker. 3. Mix well and transfer the solution to the graduated cylinder. 4. Repeat steps 1-3. PART III: Chemical & Physical Changes A. Reactions with sodium hydroxide 1. Add a small amount of copper (II) sulfate to a small test tube (enough to cover the very bottom of the tube should do). Dissolve the solid by adding enough water to fill the tube ½ to ¾ full). Gently shake to mix 2. Repeat step 1 in a second test tube, this time using baking soda (sodium bicarbonate). 3. Add a few drops of 6 M sodium hydroxide to each test tube (you ll learn what the 6 M means later in the semester). B. Reactions with zinc 1. Repeat steps 1 and 2 of Part A 2. Add a small amount of zinc powder to each test tube, and let stand for 5-10 minutes to see if a reaction occurs. C. Reactions with acetic acid 1. Repeat steps 1 and 2 of Part A 2. Add a few drops of vinegar to each test tube. Wastes Disposal. Water and solutions from Parts I and II can go down the drain with running water. Waste from Part III will be disposed of in the waste hood.

3 Name: Section: Data PART I: Accuracy & Precision A. Pipettes vs. Graduated Cylinders 10 ml pipette: Accuracy of instrument: (listed near top) Range of actual measurement (based on accuracy) Volume measured, ml (in correct significant figures) ± to Volume according to graduated cylinder, ml (first 10 ml) Volume after second 10 ml addition, ml 20 ml pipette: Accuracy of instrument: (listed near top) Range of actual measurement (based on accuracy) Volume measured, ml (in correct significant figures) ± to Volume according to graduated cylinder, ml (first 20 ml) Volume after second 20 ml addition, ml

4 B. Graduated Cylinders vs. Beakers 50 ml graduated cylinder/250 ml beaker: Volume (graduated cylinder) Volume (according to beaker) 50 ml 100 ml 150 ml 200 ml 100 ml graduated cylinder/600 ml beaker: Volume (graduated cylinder) Volume (according to beaker) 100 ml 200 ml 300 ml 400 ml What is the accuracy range given for each beaker (if none is given, assume ±5%)? 250 ml: 600 ml: What is the range of the actual measurement in each case? 250 ml: to 600 ml: to

5 PART II: Combining Volumes A. Pipettes vs. Graduated Cylinders Trial 1 Trial 2 Volume of water added, ml (according to pipette) Volume of isopropanol added, ml (according to pipette) Total volume, ml (according to graduated cylinder) B. Combining a solid with a liquids Trial 1 Trial 2 Mass of sodium chloride, g Volume of sodium chloride, ml (d = 2.17 g/ml) Volume of water, ml Volume of solution, ml PART III: Chemical & Physical Changes A. Reactions with sodium hydroxide Observations: B. Reactions with zinc Obersvations: C. Reactions with acetic acid Observations:

6 Name: Section: Post-lab questions 1.Rank the accuracy of the three instruments you used to measure volumes, from least accurate to most. 2. Given your results, explain what people mean when they say that volumes aren t additive. 3. In Part III, when you dissolved the copper (II) sulfate and sodium bicarbonate in water, were they undergoing a physical or chemical change? Explain. 4. Which compound from Part III appeared to undergo a chemical change when combined with each of the following? In each case, explain your answer. a) sodium hydroxide? b) zinc? c) acetic acid?

7 Name: Section: Pre-lab questions 1. Define the following a) chemical change b) physical change c) accuracy d) precision 2. How many significant figures are in each of the following measurements (assuming none are exact)? a) grams b) 10.0 ml c) 75 ft 3. Record the volume shown below in the correct number of significant figures? ml 4. Watch the following video on using volumetric pipettes: a) How do you get rid of any liquid in the tip that could potential contaminate your sample? b) How many rinses do they recommend? c) Why should you not do what they recommend at the 2:23 mark during today s lab?