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Reliability in Measurements

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Measurements must be Accurate & Precise.

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Accuracy is how close a measurement is to an accepted value (the book value) In other words, did you get close to the correct measurement??

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Example: Water boils at 100 C. You boil water and measure the boiling point to be 98 C. Is your measurement accurate? Accurate would have to have < 5% error. Yes, Although this value is close there is a small amount of error.

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You boil the water a second time. This time, you find the water to boil at 76 C. Are you accurate? NO!NO! You didnt get anywhere close to the accepted BP of water (100 C) Example:

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How can you tell how accurate your measurements are? How much error do you have?

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Percent Error = a calculation to determine how accurate you are It shows how much error you have

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accepted value: the value you want to get; the book value experimental value: the value YOU get in an experiment

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What do these weird lines mean in this formula? The lines are absolute value marks which means you CANNOT get a negative answer!

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What are two reasons you might not make an accurate measurement? 1. Human error 2. Machine error

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Lets Practice! 1.The accepted boiling point for a sample of astatine 350 C. A chemist boils a sample and finds the temperature to be 365 C. –What is her percent error? –Is she accurate?

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2.A student finds the mass of an object to be 19.5g. The accepted mass of the object is 12.2g. –What is his percent error? –Is he accurate?

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Precision is how close a series of measurements are to one another.

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Example: A student boils water 4 times and gets the following data: Trial 1: 65 C Trial 3: 67 C Trial 2: 65 C Trial 4: 66 C Is the student accurate? NO! The BP of water is 100 C

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Trial 1: 65 C Trial 3: 67 C Trial 2: 65 C Trial 4: 66 C Is the student precise? YES! because all the BPs were close to the same value. Precision has NOTHING to do with the accepted value!

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Stop for a moment...

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Precision can be determined by the equipment used to make the measurement

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AND getting the same measurement over and over with a small amount of error each time – thats precision!

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Which reading is more precise? 8.50 g or g g is more precise because it has more numbers These numbers are called significant figures

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sig. figs. represent precision sig. figs. include all known numbers plus one estimated number (not known for sure) example: In the number 8.503, the digits known for sure are 8, 5, and 0, but 3 is the estimated number

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IMPORTANT: If the equipment you are using is DIGITAL, the estimated digit has been done for you!!! IMPORTANT: If the equipment is NOT digital, YOU have to estimate one place past the number you know for sure!

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To find the scale of a piece of equipment Try:

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*Find the uncertainty in the measurement: 1 st : What is the scale here? the scale is 1 C 2 nd : Read instrument 87 C for sure 3 rd : Go one place PAST what we know and estimate 87.5 C

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*Find the uncertainty in the measurement: 1 st : What is the scale here? the scale is 1 C 2 nd : Read instrument 35 C for sure 3 rd : Go one place PAST what we know and estimate 35.0 C

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*Find the uncertainty in the measurement: 1 st : What is the scale here? the scale is.2mL 2 nd : Read instrument 6.6mL for sure 3 rd : Go one place PAST what we know and estimate 6.60mL

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*Find the uncertainty in the measurement: 1 st : What is the scale here? the scale is.5mL 2 nd : Read instrument 11.5mL for sure 3 rd : Go one place PAST what we know and estimate 11.50ml 1 st : What is the scale here? the scale is.5mL 2 nd : Read instrument 11.5mL for sure 3 rd : Go one place PAST what we know and estimate 11.50ml

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*Find the uncertainty in the measurement: 1 st : What is the scale here? the scale is.1cm 2 nd : Read instrument 5.1cm for sure 3 rd : Go one place PAST what we know and estimate 5.15cm

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Lets practice...

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