Name: ______________________________

Date: _____________________ Period: ___________________

Accuracy and Precision in Measurements Introduction: When your doctor prescribes a medicine, you rely upon the pharmacist to fill the prescription with tablets that contains the correct chemicals and dosages. Pharmaceutical companies take great care that each tablet manufactured contains the same amount of active ingredient (precision) and the correct amount (accuracy). A high degree of precision and accuracy are required in the manufacture of a 1 mg tablet of prazosin hydrochloride, which is used to control high blood pressure. Less accuracy and precision are needed to produce a 400 mg tablet of aspirin. I. Purpose: The experiment stresses the importance of accuracy and precision in scientific measurements and experimental results. An understanding of how accuracy and precision are determined and the difference between them is a goal of this exercise. In this experiment and all experiments in chemistry all measurements and derived values must be recorded in correct number of significant figures and include a unit. Define: Accuracy, and precision. II. Materials: Triple beam balance, paper towels, weigh boats, 50 mL graduated cylinder, 10 mL graduated cylinder, calculator, pencil, goggles, apron, water. Safety: Eye protection, clothing protection III. Procedure: 1. Obtain the mass of the empty graduated cylinders. 2. Obtain three different volumes of water and record. 3. Obtain the masses of the empty 50 mL and 10 mL graduated cylinders 4. Obtain the mass of the water and graduated cylinder. 5. Calculate the mass of just the water and record. IV. Data: Sample Data Table: Mass and Volume of Water Mass of Empty Mass of Grad. Cyl. Grad. Cyl. (g) And Water (g) Trial 1 xxxxxxxxxxxxxxxxx xxxxxxxxxxxxxxxxx Trial 2 xxxxxxxxxxxxxxxxx xxxxxxxxxxxxxxxxx Trial 3 xxxxxxxxxxxxxxxxx xxxxxxxxxxxxxxxxx

Volume (mL) xxxxxxxxxxxxxxxxxx xxxxxxxxxxxxxxxxxx xxxxxxxxxxxxxxxxxx

V. Results: Use at least 3 pieces of data in your results section. Make a statement and support with evidence. Draw the reader’s attention to patterns or anomalies.

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Name: ______________________________

Date: _____________________ Period: ___________________

VI. Analysis: Remember you must show all calculations and include units in all measurements, calculations and answers. Each calculation should have a title (For example: Volume of Density of Water Trial 3). 1. Calculate the density of the water for each trial. 3. Calculate the mean (average) density. 4. Use the mean density as your experimental result and the accepted density (1.00 g/mL) to calculate your percent error.

VII. Conclusion: Include the normal requirements for a conclusion. Questions: 1. What must be known before the percent error can be determined? 2. What are the possible sources of error in this experiment? 3. Heather and Jason determined the density of a liquid three different times. The values they obtained were: 2.84 g/mL, 2.85 g/mL and 2.83 g/mL. The accepted value is known to b 2.30 g/mL. a. Are the values Heather and Jason obtained precise? Explain. b. Are the values accurate? Explain. 4. When cutting the legs of a table to make it lower, what is most important precision or accuracy? Explain.

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