2. Science Making measurements Accuracy and Precision

3. Accuracy vs Precision 4 When you make a measurement it is often necessary to determine how good that measurement is. 4 Imagine a machine filling cans of soda. The company wants to make sure that there are exactly 12 ounces of soda in every can. 4 The company needs to gather information about the accuracy and precision of the filling machine. 1 of 9 Home

4. Accuracy vs Precision 4 Accuracy and precision tell us how good a measurement is. 4 The terms accuracy and precision often cause trouble because we tend to use them interchangeably. However, these words have very different meanings. 4 Accuracy tells how close the measurement is to the true value. Precision describes the repeatability of the measurement. 2 of 9

5. Accuracy vs Precision 3 of 9 4 A measurement is precise if it can be repeated and the same value is attained. ?Is the soda filling machine below precise?  This machine is not very precise. –It delivers a different amount of soda each time.

6. Accuracy vs Precision 4 of 9 4 A measurement is accurate if it is close to the true value. This machine is supposed to deliver 12 oz. ?Is the soda filling machine below accurate?  This machine is not very accurate either. –If it delivers a different amount of soda each time, then it can’t be very close to 12 oz.

7. Accuracy vs Precision 5 of 9  Is the soda filling machine below accurate and/or precise?  This machine is precise. –It delivers the same amount of soda each time.  This machine is not accurate. –It is not putting 12 oz in each can.

8. Accuracy vs Precision 6 of 9  Is the soda filling machine below accurate and/or precise?  This machine is precise. –It delivers the same amount of soda each time.  This machine is accurate. –It is putting 12 oz in each can.

9. Accuracy vs Precision 7 of 9 4 We said that this machine is accurate because it is delivering 12 oz of soda. But how do we know that exactly 12 oz is going into each can? 4 The only way to check the accuracy of a measurement is compare it to a standard. –Standards have known values 4 The can is the standard in this case because we know it holds exactly 12 oz.

10. Accuracy vs Precision  Define accuracy and precision 8 of 9  Accuracy describes how close a measurement is to the true value. You need a standard to determine accuracy.  Precision describes how similar the values are each time you measure.

11. Accuracy vs Precision  How could you tell if the scale in the produce department of the supermarket is accurate and precise. 9 of 9  The only way to check the accuracy of the scale is to use a standard. You need something of known weight. If you put a 1 pound package of butter on the scale and it reads 1 pound then the scale is accurate. The butter was used as a standard.  You could weigh a bunch of bananas three times. If you get the same value each time then the scale is precise. This doesn’t necessarily mean the scale is accurate. Home

12. Science Measurements You have completed “Accuracy vs Precision”

13. Denisty 4 Density can be defined as the amount of matter present in a given volume of substance. 4 Density = mass/ volume M DVDV

14. EXAMPLE 4 SOLVE 4 D= 4 M= 4 V= 4 Mercury has a density of 13.6g/mL. What volume of mercury must be taken to obtain 225g of the metal?

15. Problem A piece of copper has a mass of 57.54 g. It is 9.36 cm long, 7.23 cm wide, and 0.095 mm thick. Calculate density (g/cm 3 ).

16. SOLVE Calculate volume in cubic centimeters first. Calculate the density. (9.36 cm)(7.23 cm)(0.095 cm) = 6.4 cm 3 Note only 2 significant figures in the answer!

17. Volume Displacement A solid displaces a matching volume of water when the solid is placed in water. 33 mL 25 mL 25 mL

18. Learning Check What is the density (g/cm 3 ) of 48 g of a metal if the metal raises the level of water in a graduated cylinder from 25 mL to 33 mL? 1) 0.2 g/ cm 3 2) 6 g/m 3 3) 252 g/cm 3 33 mL 25 mL