AP Lab 1 - Measurement, Units, and Significant Figures

Transcription

1 AP Lab 1 - Measurement, Units, and Significant Figures Purpose: To understand the principles of metric measurement. To become familiar with laboratory equipment used to measure length, mass, and volume. To relate the calibration of laboratory equipment to the uncertainty in the measurements made, and to the number of significant figures in the recorded measurement. Materials: (some of these materials will be set up for your use on the side counter, not in your drawer) Ruler A 50 ml beaker 100 ml graduated cylinder (on side lab bench) Ruler B 250 ml beaker 1000 ml graduated cylinder (on side lab bench) Electronic Balance tap water 10 ml graduated cylinder (on side lab bench) Weigh boat 100 ml graduated cylinder buret in buret clamp (on side lab bench) Small rubber stopper 25 ml graduated cylinder 10 ml graduated cylinder notebook paper (for length measurement) disposable pipets Solution A Procedure: A. Linear Measurement 1. Examine the ruler marked Ruler A and note the divisions on it. What is the unit corresponding to the smallest division? How many of these units are there on the ruler? What unit do the numbers on Ruler A indicate? How many of these units are there in one meter? 2. Examine the ruler marked Ruler B and note the divisions on it. What is the unit corresponding to the smallest division? How many of these units are there on the ruler? What unit do the numbers on the ruler indicate? How many of these units are there in one meter? 3. Measure the width of a page in your lab notebook using Ruler A and again using Ruler B. Record your measurements in the data table. Be sure you include the unit with each measurement! Now measure the diameter of one of the holes in the margin of a sheet of notebook paper using each ruler and record your results in the data table. 4. Check your length and width measurements again to be sure you have included the correct number of significant figures in your measurements. The final digit in each recorded measurement should be an estimate (should contain some uncertainty). For each of the measurements you recorded, which decimal place contains the uncertainty? Is this answer different for the Ruler A measurements than for the measurements made using Ruler B? Explain why or why not.

2 B. Mass Measurement The electronic digital balances in our laboratory are very sensitive instruments. Moving the balance affects its calibration PLEASE DO NOT MOVE THE BALANCES UNLESS I TELL YOU TO DO SO!! ALWAYS USE A WEIGH BOAT, WEIGHING PAPER, OR A CONTAINER NEVER PLACE ANY CHEMICAL DIRECTLY ON THE PAN OF THE BALANCE! STEPS IN MAKING A MASS MEASUREMENT: 1. Turn the balance on. 2. Zero the balance. Wait to be sure the display says 0.00g. 3. Place a weigh boat or container on the pan and press Tare or Zero. (This step allows the balance to automatically subtract the mass of the weigh boat or container from the total mass.) 4. Add the substance to be massed into the weigh boat or container. (If you accidentally take too much chemical, DO NOT put the excess back into the stock container. Ask your teacher how to dispose of the excess.) 5. Record the displayed mass in your data table with the correct unit. (Most of the mass measurements we will make this year will use the centigram balances at your lab tables. The total capacity of most of these balances is grams and the measurement precision is ± 0.02g.) Mass the following and record the results in your data table: Large paper clip small rubber stopper C. Volume Measurement We will use several different types of glassware to measure liquid volumes primarily graduated cylinders, and burets. It is essential to understand the appropriate use for each of these types of equipment and how they differ. In general, the smaller the glassware calibration unit, the more certainty we have in the accuracy of the volume measurement. 1, On the side lab bench there are three graduated cylinders filled with different volumes of water. These cylinders are used to measure out volumes of fluids and can be read to a fairly high level of precision. The volume is determined by reading the bottom of the meniscus (for water and aqueous solutions), the curved surface that liquids make in a narrow tube. Carefully read the volume of liquid in each graduated cylinder on the side lab bench and record your answers in Table 3. DO NOT TOUCH THE CYLINDERS ON THE SIDE LAB BENCH! Be sure you are reading each meniscus at eye level. Be sure you understand the calibration of each of these cylinders before you read it. You determine the smallest division of the overall cylinder volume (e.g. 10 ml, 1 ml, 0.1 ml) to know which digit will be the first uncertain digit. In general, you can estimate to a precision of 1/10 the size of the smallest division on a measuring instrument. For example, when reading the 100 ml graduated cylinder, the smallest division marked on the cylinder is 1 ml. Therefore, you should read this cylinder out to the 0.1 ml place so that the last digit you report is an estimate (e.g ml). 2. Although beakers are not measuring instruments, many beakers do have (very approximate) volume division markings on them to be used ONLY FOR ESTIMATING VOLUMES! Do the following to see why. Place 20 ml of tap water into a 50 ml beaker (use the markings to help you estimate 20 ml) and transfer the water to a 25 ml graduated cylinder. Read the volume and record it in Table 4. Place 50 ml of tap water into a 250 ml beaker (use the markings to help you estimate 50 ml) and transfer the water to a 100 ml graduated cylinder. Read the volume and record it in Table We will use burets several times during the year to perform titrations. To refresh your memory on reading a buret, look carefully at the scale of the buret set up on the side lab bench. What volume unit do the numbers represent? What is the smallest division shown on the buret?

3 Which digit in a buret reading will be the first uncertain digit? When you take a volume reading from a buret how many places past the decimal should you read? Explain your answer. Carefully take a reading of the liquid in the buret and record it with the correct unit in Table 5. (Note that burets are always used to dispense precise amounts of liquid by recording initial and final readings and taking the difference of these readings to obtain the liquid volume dispensed. The reading you are recording here is NOT the volume of the liquid in the buret. Also, remember that we NEVER subtract a buret reading from in an attempt to calculate the true volume.) D. Density Determination Liquids It is important to remember that the data we record in the lab convey not only the numerical value and unit of the measured quantity, but also the uncertainty in the measurement (through the use of significant figures or plus-and-minus uncertainty data). In this section of the lab, you will determine the density of a solution ( Solution A ) and report the density using significant figures to indicate uncertainty. 1. Fill a new pipet with Solution A, and dry the outside of the pipet. Mass the entire unit pipet and solution on the centigram balance and record the mass. 2. Empty the Solution A from the pipet into the 10-mL graduated cylinder and record the volume using correct significant figures. 3. Record the mass of the empty pipet. 4. Calculate the density (in glml) of Solution A, observing the rules for significant figures. 5. Repeat steps #1 through #4 twice. Then determine the average density of Solution A, observing the rules for significant figures.

4 Data: Table 1: Length Measurements: Lab Notebook Paper Equipment Width of Notebook Paper Diameter of Hole in Notebook Paper Ruler A Ruler B Table 2: Mass Measurements Paper clip Rubber stopper Table 3: Graduated Cylinder Measurements (from side lab bench) Vol. in the 1 L cylinder Vol. in the 100 ml cylinder Vol. in the 10 ml cylinder Table 4: Comparison of Volumes from Beaker and Graduated Cylinder Beaker Volume Grad. Cylinder Vol. Table 5: Buret Reading Table 6: Density Determination Data Mass of full pipet (g) Trial 1 Trial 2 Trial 3 Mass of empty pipet (g) Volume of Solution A (ml)

5 Density Calculations: Mass of Solution A (g) Trial 1 Trial 2 Trial 3 Calculated Density (g/ml) Average Density of Solution A: Questions and Problems: 1. Express the width of the notebook paper in Table 1 above in the following units: m, cm, mm 2. Which ruler would give you the greatest certainty in the measurements you recorded above, Ruler A or Ruler B? Explain your choice. 3 If your lab partner told you that she had measured her pencil and the length was 14.2 cm, which ruler did she use? How do you know? 4. Is there uncertainty in the mass measurements made with the digital electronic balance? If so, in which decimal place of the mass reading would the uncertainty be indicated? 5. What is a meniscus and why is it important for measuring volumes? 6. Which of the three graduated cylinders on the side lab bench gives measurements with the greatest precision? Explain your choice. 7. Why is a beaker not an appropriate choice as a precise measuring apparatus for liquid volumes? 8. The author of a scientific article reports that he measured a liquid volume as 7.93 ml. Assuming he made the measurement properly, what was the smallest division on the measuring device he used? What two pieces of volumetric measuring glassware used in this experiment could have been used to make this measurement?