Reliability in Measurements

Measurements must be Accurate & Precise.

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?”?”

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 be exactly what the book value is. Yes, Although this value is close there is a small amount of error.

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

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

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

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

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

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

Let’s Practice! 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?

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?

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

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

Trial 1: 65°C Trial 3: 67°C Trial 2: 65°C Trial 4: 66°C 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!

Stop for a moment . . .

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

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

8.503 g is more precise because it has more “numbers” Which reading is more precise? 8.50 g or 8.503 g 8.503 g is more precise because it has more “numbers” These numbers are called significant figures

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

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!

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

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

*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

*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

*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

Let’s practice . . .