 # Scientific Measurement Investigation: Det’n of Thickness of Al foil Counted versus Measured Quantities Significant Digits and Measurement - Rules for Significant.

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Scientific Measurement Investigation: Det’n of Thickness of Al foil Counted versus Measured Quantities Significant Digits and Measurement - Rules for Significant Figures Accuracy and Precision Errors in Measurement Scientific Notation Rules for Calculations Involving Measured Quantities Investigation: Determination of the Density of Water

Investigation: Determination of Thickness of Aluminum foil

Take note of what gauge of foil you’re given: regular, heavy duty, etc. What equipment/info. do you need to find the thickness of the Al foil? square of foil mm ruler electronic balance access to PT at front of room Brainstorm... and get busy! Write your answer on the board.

Counted Quantities vs Measured Quantities How many measuring spoons are illustrated? Four. Are you sure? Yes. How many people are in this room? Do you have 100% confidence in this value? Yes.

Counted quantities have no uncertainty associated with them. If there are 15 people in a room, this may also be expressed as 15.00000000000000000000000000—etc There is an infinite number of significant figures associated with a COUNTED QUANTITY.

Measured Quantities Is it possible to use the illustrated devices to measure with absolute certainty? No. All measurements have some uncertainty.

Significant Figures and Measurement When measuring, ALWAYS ESTIMATE BETWEEN THE TWO SMALLEST SCALE DIVISIONS to obtain the estimated (uncertain) digit. eg. When using a mm ruler, estimate the tenth of a mm.

When using an ungraduated metre stick, estimate to the tenths of a metre. When measuring temperature on a thermometer calibrated every degree, we estimate the tenths of a degree.

How long is this line? ____________ the length of the line is 2.08 cm. Remember—the “8” is estimated and is therefore uncertain.

How long is this line? _____________________ The line is 4.00 cm long.

Read the temperature indicated on the thermometer to the correct number of sig figs. Ans: 32.6 o C

Significant Digits All measured quantities have some degree of uncertainty. [cf. counted quantities have no uncert.] The significant digits in a measurement include all the digits that can be known precisely plus a final digit that must be estimated. Every measured quantity MUST have ONE estimated, or uncertain, digit.

Rules for Significant Digits 1. All non-zero digits in a recorded measurement are significant. The measurements 24.7 m, 0.743 m, and 714 m all express measures of length to three significant digits.

2. Zeros appearing between non- zero digits are significant. The measurements 7003 m, 40.79 m, and 1.503 m all have four significant digits.

3. Zeros appearing in front of all nonzero digits are not significant. They are acting as place holders. The measurements 0.071 m, 0.42 m, and 0.000 099 m all have two significant digits. Sci. not’n is your friend. ie. 0.071 m = 7.1 x 10 -2 m 0.000 099 m = 9.9 x 10 -5 m

Check this...

orig. volume of coffee = 2.0 x 10 2 mL “new” vol. of coffee = 2.0 x 10 2 mL here’s why: vol. before addition = 200mL + 0.05mL “new” volume= 200.05mL Taking into account confidence limit, “new” volume = 2.0 x 10 2 mL  SAME

Accuracy How close a measurement is to the accepted value. e.g. the proximity of a dart to the “bullseye”

Precision 1. A precise instrument measures to more decimal places. e.g. a mm ruler is more precise than a cm ruler. 2. A precise set of data points are close to one another. eg. On a dartboard high precision has all darts close together, but not necessarily at the bullseye.

Errors in Measurement Systematic Error: The same error occurs over and over. eg. Taking a set of masses with an improperly calibrated balance. Random Error: Errors that cannot be accounted for.

For which type of error can we most easily correct? systematic... why? Because you make the same error each time.

A Possible Ambiguity The distance from Earth to moon is 382 000 km. How precise is this distance? We don’t know. How can we eliminate this ambiguity? Use sci not’n: eg 3.82 x 10 5 km

Scientific Notation Some examples: 6807 = 6.807 x 10 3 0.000 000 000 813 = 8.13 x 10 -10

To make it easy— Always put values in scientific notation. ie. 0.00670 cm  put in sci. not’n  6.70x10 -3 cm, or  6.70E-3 cm remember that the right-most zero is estimated, and is therefore a sig. fig.

Calculations Involving Measured Quantities Overlying Principle: A calculated value can have no more precision than the data used to calculate it. You can’t create precision. You can’t create a greater confidence level simply by doing a calculation. “You can’t make a silk purse out of a sow’s ear.”

Calculations Involving Measured Quantities 1. Multiplication and Division of Measured Quantities The measurement with the FEWEST number of significant figures that goes into a multiplication/division determines the number of sig figs in the final answer. Fewest # s.f. “in” = # sf in answer. Don’t forget to round the answer.

examples a)Find the area of a room whose dimensions are 23 m x 7.582 m. A= (23 m)*(7.582 m) = 174.386 m 2 says your calculator, or A= 1.7 x 10 2 m 2 to two sig figs.

b) Osmium, a metal, is the most dense element. Calculate the density of Os if a 12.3 cm 3 piece of Os has a mass of 277.98 g. ρ= mass/volume = 277.98 g/12.3 cm 3 = 22.6 g/cm 3 to three sig figs.

c) A vehicle travels 278 miles on 11.70 gallons of gasoline. What is the average fuel consumption in miles per gallon of the vehicle, to the correct number of sf.?

2. Addition and Subtraction of Measured Quantities Remember that the final answer can have only ONE estimated digit. e.g. Add these: 2.45m 4.5 m + 8.695 m 15.645 m. We must round this off to one estimated digit, or 15.6 m

eg. d) Here are the masses of several plums. 56.4g, 65.5g, 62.34g, 102g, 77.8g. Calculate the average mass of a plum. 56.4 65.5 62.34 102 + 77.8 364.04... con’t

364 g/5 plums = 72.8 g/plum says your calculator. To the correct number of sf, the average mass of a plum is 73 g. The least certainty was in the “units” digit of the 102 g plum. But what about the 5 plums? This is a counted quantity and therefore has infinite sf.

f) Find the sum of 3.18 and 0.01315 to the correct number of sf.

Sample Problem Using a mm ruler, determine the thickness of one sheet in your Chemistry textbook. Solution: 680 pages; thickness = 2.23 cm; How many sig figs in each of these? ∞  counted; 3 sf  measured

680 pages/2 pages·sheet -1 = 340 sheets 2.23 cm/340 sheet = 6.55882353 x 10 -3 cm/sheet = 6.56 x 10 -3 cm/sheet, or = 6.56 x 10 -2 mm/sheet

Compare to thickness of Al foil Al foil= 1.5 x 10 -2 mm; textbook sheet= 6.56 x 10 -2 mm Why is the textbook sheet’s thickness more precisely known? mass of Al foil  2 sf number of sheets  ∞ sf (counted quantity)

Rounding Off When doing a series of calculations involving measured quantities, round off only the final answer. Allow your calculator to carry a few more sig. digs throughout the (intermediate) calculations to avoid compounding errors due to rounding.

Rounding “fives” Look at these numerals: 1 2 3 4 5 6 7 8 9 What will happen if we always round a 5 “up” in a large set of data? Average will be skewed slightly upward. How can we overcome this problem? We need to round a 5 up half the time, and down the other half. But how can we easily do this?

Rounding 5s (con’t) If the digit before the 5 to be rounded is odd, round “up”. e.g. 6.75 g  6.8 g If the digit before the 5 to be rounded is even, do not round “up”. e.g. 3.45 m  3.4 m

If there are digits after the 5 that needs to be rounded, always round up. This makes sense, since we’re rounding something that’s greater than 5. e.g. 4.6851 g (to 3 s.f.)  4.69 g

Investigation: Determination of the Density of Water For each pair of students: Use the equipment provided to determine the density of tap water. Write your raw data and your calculated density of water on the board. Report your answer to correct number of significant figures.

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