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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 100C. You boil water and measure the boiling point to be 98C. 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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Example: You boil the water a second time. This time, you find the water to boil at 76C. Are you accurate? NO! You didn’t get anywhere close to the accepted BP of water (100C)

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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? Human error Machine error

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

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**A student finds the mass of an object to be 19. 5g**

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: 65C Trial 3: 67C Trial 2: 65C Trial 4: 66C Is the student accurate? NO! The BP of water is 100C

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**Trial 1: 65C Trial 3: 67C Trial 2: 65C Trial 4: 66C**

Is the student precise? YES! because all the BP’s 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 – that’s precision!

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**8.503 g is more precise because it has more “numbers”**

Which reading is more precise? 8.50 g or g 8.503 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:**

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

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***Find the “uncertainty” in the measurement:**

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

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***Find the “uncertainty” in the measurement:**

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

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***Find the “uncertainty” in the measurement:**

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

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***Find the “uncertainty” in the measurement:**

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

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Let’s practice . . .

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Accuracy vs. Precision Measurements need to accurate & precise. Accurate -(correct) the measurement is close to the true value. Precise –(reproducible)

Accuracy vs. Precision Measurements need to accurate & precise. Accurate -(correct) the measurement is close to the true value. Precise –(reproducible)

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