Lab #3 ph and Buffers

Transcription

1 Page1 Lab #3 ph and Objectives: Learn to construct a proper data table and line graph Understand how the ph scale works Use a ph meter to measure the ph of common household substances Understand the meaning of acidic and basic Collect experimental data to observe and understand the effects of buffers Equipment: Goggles, gloves, aprons, and proper shoes. Pre-laboratory Activity: Follow directions below in Part I to properly construct data tables. 1. Read Experiment 1 (Part III. B. on page 5). Construct a data table on page 7 of lab report. 2. Read Experiment 2 (Part III.C. on page 5). Construct a data table on page 8 of lab report. I. Scientific Method: Data Tables and Graphs A. Data tables Data are information or experimental values that are collected. We will be collecting data (singular: datum) by doing experiments. Follow the guidelines below for creating a data table: 1. Number and title the table. Give a number to the table (corresponding to where it appears in the report) with a title that describes the contents and mentions both variables. The independent variable is the factor that is being controlled or modified to determine how it affects the dependent variable in an experiment. Including the units in the title can increase clarity. The title should go at the top of the table. 2. Label the dependent and independent variables on the table. Construct your table so that the data collected go in the empty spaces. Include the units of measurement that you will be using in the table. Make sure to include enough rows and columns to accommodate all of your data. See the example below: Table 1. Diffusion experiment. The diameter (mm) of diffusing substances on an agar plate over time (minutes). Diffusing Substance Methylene blue (mm) Time 0 Time 5 minutes Time 10 minutes Time 15 minutes Time 20 minutes Potassium Permanganate (mm)

2 Page2 B. Graphs Graphs can be used to represent data visually to make it easier to see trends in the data and draw conclusions. There are many types of graphs- a few examples are shown here. In order to create a graph using your data, you need to do the following steps: 1. Decide what type of graph you will be creating that best describes the data. We will be using line graphs most frequently in laboratory. 2. You must number the figure. All photos, drawings, and graphs need to be numbered in the order in which they appear in the laboratory report. 3. Give the graph a name or title that describes the contents and mentions both variables, independent and dependent. When possible, include the units in the title to increase clarity. The title should go at the top or bottom of the graph. 4. You must describe and label the dependent and independent variables on the graph. Typically, the independent variable will be on the X axis, and the dependent variable will be on the Y axis. Include the units of measurement on the graph for each axis. 5. Construct your graph so that the data collected is spread out enough. Look at the range of the data so that you can choose your increments for your x and y axis. Using the data shown in this table, a graph was constructed and is shown on the next page. Table 1. The number of students attending laboratory over time (in weeks) Week # of students

3 Page3 LABORATORY REPORT ACTIVITIES : 1. Using the grid provided in your lab report, construct a graph for the Experiment 1 data you collect. Use the data in the table you constructed. Be sure to label all the parts described on page 2 and shown in the graph above. 2. Using the grid provided in your lab report, construct a second graph for the Experiment 2 data you collect. Use the data in the table you constructed. Be sure to label all the parts described on page 2 and shown in the graph above. II. Introduction to ph: ph is a measure of the concentration of hydrogen ions (H + ) in a solution. The ph scale ranges from Acidic is ph 0-6, basic (or alkaline) is ph 8-14, and ph 7 is neutral. Define the following terms: 1. acid 2. base

4 Page4 3. salt 4. buffer 5. ph scale Equipment: a) ph meter The ph meter must be carefully used. The cap must be removed and the probe at the bottom submerged in the liquid of choice. The on/off switch is on the top and the digital display on the front. It is normal for the ph to fluctuate slightly while reading. The probe should not be submerged above the immersion line and must be rinsed after each use by using a distilled water squeeze bottle and a Syracuse (round, flat bottomed) dish to collect the fluid. The probe should be rinsed with buffer prior to being recapped at the end of the laboratory. b) Hot plate (with magnetic stirrer) The hot plate portion will not be used today so you must ONLY turn on the stirrer. A magnetic stir bar will be placed in the beaker to stir the solution as you add the acid. Do NOT let it stir too fast such that the solution splashes. The magnetic stir bar must be removed (you may use a magnetic retrieval wand) when the solution is disposed. Hot plates must be supervised at all times when in use. beaker stir bar hot plate

5 Page5 III. The action of a buffer A) Calibration of the ph meter: Using a 250 ml beaker, obtain 100 ml of the ph 7.0 buffer. Measure the ph of the buffer. Record the results in the lab report in the table you have constructed (see IA.). It should read 7.0 (from ). If it is not correctly calibrated, ask your instructor for help. Keep the buffer for Experiment C. B) Experiment 1-0.5M HCl added to water 1) Using a 250 ml beaker, obtain 100 ml of tap water. Measure the ph of the water and record it in the table that you created for Experiment 1 data in the lab report (0 drops). Place a magnetic stir bar in the beaker, sliding it down the side. Place the beaker on the magnetic stirrer and begin stirring. Make sure not to turn on the heat! 2) Using the100ml beaker, obtain about 40 ml of 0.5 M HCl from the side bench and carefully transport it back to your bench. Do not pour any HCl back into the main bottle. Your instructor will give you the ph of the HCl do not measure it yourself. 3) Using a disposable pipette, add 10 drops of HCl at a time and record the ph in your Experiment 1 table after each 10 drop measure. 4) After all 100 drops have been added, properly dispose of the water/hcl mixture in the 250 ml beaker. Be careful not to discard the stir bar. Keep the HCl in the 100 ml beaker for Experiment 2. 5) Plot the data on the grid provided in the laboratory report to produce a line graph. Follow the directions in I.B. for labelling your graph. C) Experiment 2 0.5M HCl added to ph 7.0 buffer AAA Always Add Acid to water never the opposite! 1) Using a 250 ml beaker, obtain 100 ml of buffer. Measure the ph of the buffer and record it in the table you created for Experiment 2 data of the lab report (0 drops). Place a magnetic stir bar in the beaker, sliding it down the side. Place the beaker on the magnetic stirrer and begin stirring at setting 5. 2) Using a disposable pipette, add 10 drops of HCl from your small beaker (same as the one you used in Experiment 1). Record the ph in the Experiment 2 table after each 10 drop measure. 3) Continue adding 10 drops at a time until you have reached 300 or the ph ceases to change after 4 consecutive readings. Properly dispose of the buffer mixture in the beaker. Again, be careful not to discard the stir bar.

6 Page6 4.) Plot the data on the grid provided in the laboratory report to produce a line graph. Follow the directions in I.B. to label your graph. IV. The ph of common household substances 1) BEFORE you make any actual measurements, use Table 3 in your lab report to formulate a hypothesis about which substances in the list will be acidic (A), basic (B), or neutral (N). 2) Using your ph meter, measure the ph of the substances in the beakers on the laboratory bench. Take your Syracuse dish and wash bottle with you to rinse between samples. Do not contaminate the samples. 3) Record the ph measurement of each substance in Table 3 of your lab report. Then be sure the ph meter is properly cleaned, turned off, and put away. 4) Answer the questions in the laboratory report. V. Proper Disposal of Waste Materials 1. Chemicals from the beakers should be placed in the special disposal container as directed by your instructor. 2. Chemicals in the Syracuse dish used to wash the ph meter can be placed down the sink drain. 3. Disposable droppers can be placed in the regular trash.

7 Page7 Laboratory Report # 3 - ph & Name: I. A. Construct a data table for Experiment 1 (USE A RULER NEATNESS COUNTS!) Follow the directions in part I. III. A Calibration of ph meter ph of 7.0 buffer read by ph meter: Did the meter require calibration? Y / N? If yes, the new buffer reading: III. B. Experiment 1-0.5M HCl added to water ph of the 0.5M HCl: (as measured by your instructor) 1. How many drops did it take for the ph of the water to drop by at least 1.0 ph unit (from ph 7 to ph 6)? 2. How many drops did it take for the ph of the water to reach ph 3.0?

8 Page8 III. C. Construct a data table for Experiment 2 (use a ruler neatness counts!) Follow the directions in Part I. 1. How many drops did it take to reduce the ph of the buffer by at least 1.0 ph unit (from ph 7 to ph 6)? 2. What was the lowest ph that the buffer reached? 3. How did the lowest ph of the buffer compare to the ph of the HCl? Analysis of Data from Experiments 1 and 2 1. How did the water and the buffer differ in their response to HCl being added? 2. How do Experiments 1 and 2 demonstrate the function of a buffer? (Refer back to what a buffer is supposed to do)

9 3. Create a line graph of Experiment 1 data below (see part I for directions.) Be sure to include a title and properly label the X and Y axes.

10 4. Create a line graph of Experiment 2 data below (see part I for directions.) Be sure to include a title and properly label the X and Y axes.

11 Page11 IV. The ph of Common Household Substances A. Record Hypothesis first: A= acidic (ph 0-6) B = basic (ph 8-14) N = neutral (ph 7) B. Record Actual ph Data Table 3. The ph of Common Household Substances Substance Hypothesis (A,B, N) Actual ph Soda (cola) Coffee Glass cleaner Baking soda Milk Tap water Vinegar Tea C. Discuss whether or not the data supported / did not support your hypothesis. In your discussion, use specific information including: 1. which predictions were right or wrong 2. how many were right and how many were wrong 3. what percentage accuracy you had overall DISCUSSION (use complete sentences and be thorough):