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1.07 Accuracy and Precision

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1.07 Accuracy and Precision

Concepts

Standards:

- MA.912.S.1.2—Determine appropriate and consistent standards of measurement for the data to be collected in a survey or experiment.
Objectives:

- Distinguish between Accuracy and Precision
- Determine the number of significant figures in a measurement.
- Read and record measurements with the correct number of significant figures.

Identify the following as accurate or precise.

1. 2. 3.

- Not accurate or precise.
- Accurate and precise
- Precise but not accurate.
*Same spot but not on the bullseye

If the window measures 47 cm exactly

Are the measurements accurate/precise?

If the window measures 47 cm exactly

Are the measurements accurate/precise?

Carpenter 1: has measurement’s with great variety.

Neither accurate or precise.

Carpenter 2: precise, accurate if the true length is around 47.0 cm.

- Suppose you were asked to measure the length of a pencil using a ruler in cm?
- How do you measure accurately and precisely?

- I might say 6.7, you might say 6.8, your friend might say 6.75 cm. Who is right? How do we show consistency in the way all scientists read and record measurements?

- Use significant figures (sig figs for short) in 3 ways:
1. You will make measurements and report them to others. Determine the number of sig figs by estimating one digit past the smallest measurement, or graduation, on the measuring tool.

Ex. 1: The graduations go in 10 degree increments. So, you can know the temperaturefor certain to the tens place and can estimate in the ones place. Estimate that the temperature is 6°C, 7°C, 8°.

Ex. 2: The graduations on this thermometer mark off every one degree. You can measure the temperature to the ones place for certain and can estimate to the tenths place. You may read this thermometer as 3.6°C, 3.7°C, 3.8°.

Ex 3: The graduations on this thermometer mark off every tenth of a degree (o.1 increments).You can know the temperature to the tenths place for certain and can estimate to the hundredths place. You may have read this temperature as 0.69°C, 0.70°C, 0.71°.

2. You will interpret the measurements reported by others. Data provided will show the instrument used because you know that scientists always estimate one digit past the smallest graduation on a measuring tool.

3. You will need to keep track of sig figs when measurements are used in calculations.You must have correct values in measurement to use correct # of sig figs in calculations.

- The results of the calculations are not allowed to appear more or lessaccurate than the original measurements used.
- Follow simple rules when multiplying, dividing, adding, or subtracting helps make sure that all results are represented with the appropriate amount of reliability.

- Only given measurements affect the number of sig figs allowed in the final answer. Conversion factors or equivalences don’t affect it. (Ex: 1 m= 1000 mm)
- If you are only given one measurement, the total number of sig figs in that measurement equal the total number of sig figs allowed in your final answer.
- If you are given more than one measurement, the final answer must be rounded to the same total number of sig figs as the measurement that has the least.

Example problems

- Convert 72.0 cm to the unit dam.
- Calculate the density of an object that has a mass of 104.5 g and a volume of 64.0 mL.

Example problems

- Convert 72.0 cm to the unit dam.
- Calculate the density of an object that has a mass of 104.5 g and a volume of 64.0 mL.
D = mass / volume

Density = 104.5 g / 64.0 mL

Density = 1.6328 g / mL

**Round to 3 sig figs b/c 64.0 mL only has 3

Final answer 1.63 g /mL

10^-2 m

1 dam

72.0 cm

= 0.0720 dam

10^1 m

1 cm

- The final answer cannot have more places after the decimal than any of the given measurements.
- The final answer cannot have a final digit, which represents the uncertain or estimated place, farther to the right than any of the final digits in the measurements used.

Example problems

- Add 101.5g + 17.86 g
- Subtract 101.5g - 17.86 g

Example problems

- Add 101.5g + 17.86 g
= 28.36 g

Actual answer w/ correct Sig Figs can only have 1 place after decimal.

28.4 g

2. Subtract 101.5g - 17.86 g

83.64 g , Actual answer can only have 1 place after decimal

83.6 g

- It is sometimes necessary to round your answer or add zeros to the end of the answer to give it the proper number of sig figs.

Example problems

- Calculate the density of an object that has a mass of 101.3 g and a volume of 49.5 mL.
Density = mass/ volume

Density = 101.3 g / 49.5 mL

Density = 2.046 g/mL

Final answer can only have 1 place after decimal and no more than 3 sig figs, whatever is less. Round down.

** 2.0 g/mL

- If a beaker containing a sample of powder has a mass of 65.09 and the clean, empty beaker has a mass of 54.69 grams, what is the mass of the powder?
65.09 – 54.69 g = 10.4 g but must round to 2 decimals places after decimal, so add a zero.

**10.40 g

89.9 mL

27.79 mL

54 mL

32.81 mL

55.8 mL

45 mL

Lab worksheet link: https://sites.google.com/site/chemistryflvs/Tutorials/lab-report-files/1_07AccuracyandPrecisionLabWorksheet.doc?attredirects=0&d=1

or go to the Chemistry Resource Center and click Blank Lab reports.