CH 2: Scientific Measurement Renee Y. Becker CHM 1025 Valencia Community College 1

Measurements Measurement – A number with an attached unit Examples: 15 inches 3 cups 36 cm Every measurement must include units!! 2

Example 1: Measurements In the following, what are the measurements and what are the units? a) 12 trees b) 2.3 mm c) 100 ¢ 3

Measurements Instrument – A device for recording a measurement Examples: Ruler (length), electronic balance (mass), Graduated cylinder (volume) 4

Uncertainty An instrument may give a very sensitive reading, but EVERY measurement has UNCERTAINTY No measurement instrument is perfect and neither is the person using it 5

Example 2: Length Measurements If we need to measure the length of this candycane, which ruler should we use? Why? 6

Mass – Measure of the amount of matter it possesses – Measured by a balance – Not affected by gravity – Typical units: kilogram (kg), gram (g), pound (lb), ounce (oz) Weight – Force exerted by gravity on an object 7

Balances 8

Example 3: Mass Would you have the same mass on the moon as on Earth? Why? Would you have the same weight on the moon as on Earth? Why? 9

Volume – The amount of space occupied by a solid, gas, or liquid – Graduated cylinder, pipet, buret, volumetric flask, syringe – Typical units: milliliter (mL), Liter (L), centimeter cubed (cm 3 ), quart (qt), gallon (gal), 10

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Buret 12

Accuracy, Precision, and Significant Figures in Measurement Accuracy is how close to the true value a given measurement is. Precision is how well a number of independent measurements agree with one another. 13

Accuracy, Precision, and Significant Figures in Measurement Significant Figures are the total number of digits in the measurement. The results of calculations are only as reliable as the least precise measurement!! Rules exist to govern the use of significant figures after the measurements have been made. 14

Accuracy, Precision, and Significant Figures in Measurement Rules for Significant Figures: – Zeros in the middle of a number are significant – Zeros at the beginning of a number are not significant – Zeros at the end of a number and following a period are significant – Zeros at the end of a number and before a period may or may not be significant. 15

Example 4: Significant Figures How many Sig. Figs ? a) b) c) 36,450 d) e)

Accuracy, Precision, and Significant Figures in Measurement Rules for Calculating Numbers: – During multiplication or division, the answer can’t have more sig figs than any of the original numbers. 17

Example 5: Significant Figures a)238.5 x 79 = b)12 / = c) x 0.29 = d) / = 18

-During addition or subtraction, the answer can’t have more digits to the right of the decimal point than any of the original numbers. Accuracy, Precision, and Significant Figures in Measurement 19

Example 6: Significant Figures a) = b) = c) = d) = 20

Accuracy, Precision, and Significant Figures in Measurement Rules for Rounding Numbers: – If the first digit removed is less than 5 - round down – If the first digit removed is 5 or greater- round up – Only final answers are rounded off, do not round intermediate calculations 21

Example 7: Rounding Round off each of the following measurements (a) L to four significant figures (b) K to three significant figures (c) kg to four significant figures 22

Example 8: Accuracy & Precision Which of the following is precise but not accurate? 23

Scientific Notation Changing numbers into scientific notation – Large # to small # – Moving decimal place to left, positive exponent 123,987 = x 10 5 – Small # to large # – Moving decimal place to right, negative exponent = 2.39 x Correct scientific notation: #.#### x 10 n 24

Scientific Notation 25 How to put into calculator??

Example 9: Scientific Notation Put into or take out of scientific notation a)87542 b) x c)0.784 d)2.78 x 10 6 e)

Significant Figures When we count something, it is an exact number. – It has an infinte number of significant figures 27