Accuracy and Precision in Measurements

Chemistry- Laboratory #1

Name: ______Partner: ______

ACCURACY AND PRECISION IN MEASUREMENTS Introduction: In this experiment, the volume of the graduated cylinder up to the 50-mL graduation will be calculated using the measurements of its internal diameter and height to the graduation. The mass of measured volume of water will be calculated from its density and then determined by weighing. The error and percentage error in each of the experiment will be calculated on the basis of the accepted value. Experimental error = observed value - accepted value % Error = (accepted value-experimental value) x 100 accepted value The sign of the experimental error and the percentage error may be either positive (the experimental result is too low) or negative (the experimental result is too high).

Safety Alert/MSDS: Before beginning part of the experiment, take the necessary safety precautions. Wear safety goggles, and apron. It is important to use good safety techniques while conducting experiments.

Objectives: 1. 1. To learn the importance of accuracy and precision in scientific measurements and the difference between them. 2. 2. To calculate values from experimental measurements. 3. 3. To understand percentage error.

Materials List: Balance 100 mL and 10 mL graduated cylinders Celsius thermometer 100-mL beaker ruler- metric Metal shot (aluminum, copper or lead) Water

Pre Lab Questions: 1. 1. What is the formula for the calculation of density? ______2. 2. What does percentage error mean? What is the formula used to calculate this quantity? ______Procedure 1: 1. 1. Examine the centimeter scale of the plastic ruler. What are the smallest divisions? To what fraction of a centimeter are you expected to be able to make a measurement with such a ruler? 2. 2. Using the ruler, measure the inside diameter of the 100-mL graduated cylinder. Similarly, measure the inside height of the cylinder to the 50-mL graduation. Record these measurements in Data Table 1.

Data Table 1 Inside diameter of graduated cylinder cm Inside height of graduated cylinder (h) cm Chemistry Honors- Laboratory #1 Name: ______Partner: ______Date: ______Period: ______

Procedure 2: 1. 1. Examine the gram scale of the triple-beam balance. What are the smallest divisions? ______2. 2. Using the balance, determine the mass of the dry 100-mL cylinder. Record the mass in Data Table 2. Fill the beaker half-full of water and determine its temperature to the nearest degree. Look up the density of water for this temperature and record both the temperature and water density. Fill your graduated cylinder to some value between 10 and 15 mL; accurately read and record the volume. Determine the mass of the water plus the cylinder. Then, record this value in Data Table 2. Save the water in the graduated cylinder for use in Procedure 3.

Data Table 2 Mass of empty graduated cylinder g Temperature of water c Density of water g/cm3 Volume of water mL Mass of graduated cylinder + water g

Procedure 3: 1. 1. Add a sufficient quantity of the assigned metal shot to the cylinder containing the water (from procedure 2) to increase the volume by at least 5 mL. Determine the volume and then the mass of the shot, water, and cylinder, and record your measurements in Data Table 3. 2. 2. At the end of the experiment, all the equipment back in the lab drawer should be clean, dry and arranged in an orderly fashion. Ask your teacher for the proper waste container for the metal shot. Wash your hands before leaving the lab.

Data Table 3 Volume of water (from Procedure 2) mL Mass of water + graduated cylinder g Volume of metal and water mL Mass of metal + water + graduated cylinder g

Calculations: Show your calculations. Place your answers in the appropriate Calculations Table.

Part I: a. Calculate the volume of the cylinder to the 50 mL graduation (v = 3.14 (r2 )(h). b. Assume the accepted value is 50 cm3. Calculate the error and percentage error.

Calculations Table Volume of graduated cylinder (computed) cm3 Error cm3 Percentage error %

Part 2: a. Calculate the mass of the water as determined by the balance. b. Calculate the mass of the water from its measured volume and its density (m= d X V). c. Using the mass determined by the use of the balance as the accepted value, calculate error and percentage error.

Calculations Table Mass of water (observed) g Mass of water (calculated: m= D x V) g Error g Percentage error %

Part 3: a. Use volume displacement to determine the volume of the metal. b. Determine the mass of the metal. c. Calculate the density of the metal. d. Look up the specific gravity (density) of the metal in a handbook. Then calculate error and percentage error for the density of the metal shot you determined in Ó.

Calculations Table 3 Volume of the metal cm3 Mass of metal g Density of metal (computed) g/cm3 Density of metal (accepted) g/cm3 Error % Percentage Error %

Post-Lab. Questions: 1. What value of a measurement must be available if the accuracy of a measurement is to be determined?

2. What are the possible sources of experimental errors in this experiment?