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Name: _____________________________________________ Date: __________ Accuracy, Precision, and Measurements Introduction It would be hard to overstate the importance of taking proper measurements in the chemistry laboratory. The goal of most of the experiments that you will perform this year is to obtain accurate, precise data. You will use your data to perform calculations and formulate conclusions about chemical principles. Obviously, these calculations and conclusions can only be as accurate as the measurements that they are based on. For this reason, you must take each measurement to the maximum accuracy allowed by the instrument you use. The accuracy of each measurement is limited by two things: the limitations of the instrument and the care of the experimenter. You must also make sure that you don’t accidentally claim that a measurement or calculation is more accurate than it actually is. Purpose The objectives of this lab are to: Measure each physical quantity to the maximum accuracy allowed by the appropriate instrument. Record measurements and calculations with the appropriate units from the international system of units. Calculate volumes and densities to the appropriate number of significant digits. Materials electronic balance 100 mL graduated cylinder 50 mL buret 150 mL beaker distilled water ring stand buret clamp Procedure Measuring Volume Using a Graduated Cylinder 1. Determine the mass of a 100 mL graduated cylinder and record the value in Table 1. Remember to include appropriate units and an estimation digit for the measurement. 2. Obtain about 100 mL of distilled water in a clean Erlenmeyer flask. Determine and record the temperature of the distilled water. 3. Fill the graduated cylinder with distilled water so that the meniscus of the water level lines up with the 25 mL calibration mark of the cylinder. 4. Dry the outside of the graduated cylinder. 5. Determine the mass of the graduated cylinder and water and record the value in Table 1. Remember to include appropriate units and an estimation digit for the measurement. 6. Determine the actual amount of water placed in the graduated cylinder and record the value in Table 1. Remember to include appropriate units and an estimation digit for the measurement. Measuring Volume with a Beaker 1. Clean and dry a 150 mL beaker. 2. Determine the mass of the beaker and record the value in Table 2. Remember to include appropriate units and an estimation digit for the measurement. 3. Obtain about 100 mL of distilled water in a clean Erlenmeyer flask. Determine and record the temperature of the distilled water. 4. Fill the beaker with distilled water to the 50 mL calibration mark of the beaker. 5. Dry the outside of the beaker. 6. Determine the mass of the beaker and water and record the value in Table 2. Remember to include appropriate units and an estimation digit for the measurement. 7. Record the amount of water placed in the beaker. Remember to include appropriate units and an estimation digit for the measurement. Measuring Volume with a Buret 1. Clean and dry a 150 mL beaker. 2. Determine the mass of the beaker and record the value in Table 3. Remember to include appropriate units and an estimation digit for the measurement. 3. Determine and record the initial liquid level in the buret. Remember to include appropriate units and an estimation digit for the measurement. 4. Place the 150 mL beaker under the stopcock of the buret. 5. Open the stopcock of the buret and run water into the beaker until approximately 25 mL of water has been dispensed. 6. Determine and record the final liquid level in the buret. Remember to include appropriate units and an estimation digit for the measurement. 7. Determine and record the mass of the beaker and water. Remember to include appropriate units and an estimation digit for the measurement. 8. Determine and record the temperature of water in the beaker. Data Analysis For each problem below, show your work in your laboratory notebook. Record your answer in the data table. Remember to include appropriate units and the correct number of significant figures with your answer. 1. Calculate the mass of water in the graduated cylinder. Record your answer in Table 1. 2. Using the mass of water and the density of water at the measured temperature, calculate the volume of water actually in the graduated cylinder. Record this answer in Table 1. Remember, Density = mass volume 3. Calculate the difference in the observed volume and the calculated volume for the graduated cylinder. Record your answer in Table 1. 4. Calculate the mass of water in the 150 mL beaker. Record your answer in Table 2. 5. Calculate the actual volume of water in the 150 mL beaker. Record your answer in Table 2. 6. Calculate the difference in the observed volume and the calculated volume for the graduated cylinder. Record your answer in Table 2. 7. Calculate the mass of water transferred from the buret. Record your answer in Table 3. 8. Calculate the volume of water transferred from the buret. Record your answer in Table 3. 9. Calculate the actual volume of water transferred from the buret. Record your answer in Table 3. 10. Calculate the difference in the observed volume and the calculated volume for the buret. Record your answer in Table 3. Conclusion 1. Based on the data collected in this experiment, which piece of volumetric glassware is the most accurate, the graduated cylinder, the beaker, or the buret? Use data you collected in this lab to justify your answer. 2. Which piece of volumetric glassware allowed you to make the most precise measurements? How can you tell? Table 1. Determining the Accuracy of a Graduated Cylinder Mass of empty 100 mL graduated cylinder Mass of cylinder + water sample Mass of water in cylinder Observed volume of water in cylinder Temperature of water in cylinder Density of water at this temperature Calculated volume of water transferred Difference between observed and calculated volume Table 2. Determining the Accuracy of a Beaker Mass of empty 150 mL beaker Mass of beaker + water sample Mass of water in beaker Observed volume of water in beaker Temperature of water in beaker Density of water at this temperature Calculated volume of water transferred Difference between observed and calculated volume Table 3. Determining the Accuracy of a Buret Mass of empty 150 mL beaker Initial liquid level in buret Final liquid level in buret Volume of liquid transferred Mass of beaker + water sample Mass of water in beaker Temperature of water in beaker Density of water at this temperature Calculated volume of water transferred Difference between observed and calculated volume