Scientific Chemical Fundamentals Measurement Conversions Calculations Dr Ron

Scientific & Chemical Fundamentals Measurement, Conversions & Calculations Dr. Ron Rusay Spring 2008 © Copyright 2003 -2008 R. J. Rusay

Measurement & Units SI units & common units in General Chemistry (Lab Manual pp. 139 -142) • Quantitative vs. Qualitative • MASS (Chem: gram; SI: kg) • LENGTH (Chem: cm & others; SI: m) • TEMPERATURE (Celsius & Kelvin; SI: K) • VOLUME (Chem: m. L; SI: Liter) • CHEMICAL AMOUNT: Mole (mol) © Copyright 1998 -2008 R. J. Rusay

Units of Measure

Mass and Volume Measurements: Refer to pp. 5 -8

Mass Determination (Weighing Devices: Balances)

Volumes of regular shapes V=s 3 h V=lxwxh

Volume

Liquid Measurement Tools

Numbers & Measurement The Importance of Units ð Measurement - quantitative observation consisting of 2 parts • Part 1 - number • Part 2 - unit ð Examples: • 20 grams • 6. 63 joules / second

Scale: Size & Comparison ð Macroscopic vs. Microscopic ð IBM financed Video: http: //www. wordwizz. com/imagendx. htm How would you compare your lifespan? . . to that of a dog? …. to the age of the earth? …How about the age of mankind to that of all life? . . the age of industrialized mankind to the age of mankind?

Graphic Comparisons

Powers of Ten: Scale

Language describes scale (prefixes) Shorthand Prefixes How many zeroes does yotta have?

Commonly used prefixes in Chemistry These should be known from memory.

Commonly used prefixes in Chemistry Attosecond spectroscopy = 10 -15 x 10 -3 seconds Science, 317, 765 -775, (2007) “The Electron Stopwatch”

QUESTION Conveniently, a U. S. nickel has a mass of approximately 5 grams. If you had one dollar’s worth of nickels what would be the mass of the nickels in milligrams? 1. 2. 3. 4. 100 milligrams 50 milligrams 1, 000 milligrams 1000 milligrams (mg) = 1 gram (g)

ANSWER Choice 4 shows the correct conversion. After determining that 20 nickels make up one dollar, then one dollar’s worth of nickels would have a mass of 100 grams. Next, the conversion between grams and milligrams can be performed by multiplying by 1, 000 (because there are 1, 000 milligrams per gram. ) Section 1. 3: Units of Measurement

Scientific Notation & Significant Digits Scientific Notation: A single digit followed by a decimal and a power of ten. Examples: 2, 345 m. L and 0. 002340 g 2, 345 m. L = 2. 345 x 10 3 m. L 0. 002340 g = 2. 340 x 10 -3 g

Numbers • Expressing a number correctly is determined by the method used in the measurement! • How many numbers should I include? Significant Digits (Figures) Consider: the exactness of the measured value • Short Hand expression translates the number: Scientific Notation © Copyright 1998 -2008 R. J. Rusay

What is the length of the rod? Different measurement tools give different numbers: Which ruler is better? ? cm 4. 2 - 4. 3 cm ? cm 4. 24 - 4. 25 cm

What is the diameter of a circle? All measuring devices are not the same, and the values (numbers) that come from them indicate their limitations. Is there a better instrument to use other than a ruler?

What does each line represent? 1 m. L What can be estimated? O. 1 m. L

Measurement Assignment http: //chemconnections. llnl. gov/General/Chem 120/volume 1. htm

Temperature Scales Relative to Water

“Normal” Body Temperature

QUESTION Dr. R. walks into class and claims, “It is very cold in here today. It feels like 242 K. ” If that were the temperature, would you agree that you would feel cold? What would that be in Celsius degrees? 1. 2. 3. 4. I agree, that would be 31°C. I agree, that would be – 31°C. I do not agree, that would be 31°C. I agree, that would be – 31. 15°C.

ANSWER Choice 2 provides a correct (very) cold temperature. The formula to use is K = °C + 273. 15. However this must be rearranged slightly to yield K – 273. 15 = °C. Since this is a subtraction, the correct value would have no numbers beyond the decimal point because 242 does not have numbers beyond the decimal point. Section 1. 7: Temperature

Temperature

Reporting Numbers Rules for Significant Digits (Figures) ð Nonzero integers always count as significant figures. ð 3456 g has how many sig figs? ð 4 sig figs. • Expressed in scientific notation? 3. 456 x 10 3 g

Reporting Numbers Rules for Significant (Digits) Figures ð Exact numbers (unit, conversion or scale factors) can have an infinite number of significant figures. ð 1 liter = 1, 000. ml, exactly ð 1 inch = 2. 54 cm, exactly

Zeros « Leading zeros do not count as significant figures. ð 0. 0486 m. L has how many sig figs? ð 3 sig figs. • Number expressed in scientific notation? 4. 86 x 10 -2 m. L

Zeros ð ð ð Captive zeros always count as significant figures. 16. 07 cm has how many sig figs? ð 4 sig figs. Number expressed in scientific 1. 607 x 10 1 cm notation?

Zeros ð ð Trailing zeros are significant only if the number contains a decimal point. 9. 300 kg has how many sig figs? ð 4 sig figs. • Number expressed in scientific 9. 300 kg notation?

QUESTION Which one of the following does NOT represent a result with four significant digits? 1. 2. 3. 4. 0. 07100 0. 7010 0. 0710

ANSWER Choice 4 only has three significant digits. Note that the lone zero in front of the decimal point is not based on any measurement and the next zero serves only as a place holder not as a measurement. Section 1. 5: Significant Figures and Calculations

Mathematics & Arithmetic • Relative to method(s) of measurement • Short Hand expression: Scientific Notation • Numbers : How many to include? Quantitative vs. Qualitative • Addition/Subtraction. . . • Multiplication/Division. . . • What is “significant”? . . . Rounding Off • http: dbhs. wvusd. k 12. ca. us/Sig. Figs. Fable. html © Copyright 1998 -2007 R. J. Rusay

Systematic Problem Solving Dimensional/Unit Analysis: Conversions Workshop pp. 145 -149 How many m. L of milk are in a 1/2 gallon carton? 0. 50 gal 1 gal = 4 qt ? m. L 1 qt = 946 m. L 0. 50 gal | 4 qt | 946 m. L = ? m. L | 1 gal | 1 qt

Complete the following Units & Conversions Number 13, 000, 000 yrs. Scientific Notation 10 yrs 1. 3 x 10 ________ 546 ______ m. L 5. 46 X 10 2 m. L _______ 0. 845 ______ kg __8. 45 x 10 -1 kg___ Named unit __? gigayears 13 Gyrs 0. 546 Liters 0. 546 L _? grams__ 845 g

Computational Rules • Addition/Subtraction: Answer expressed • to the least number of decimal places of the figures in the process Multiplication/Division: Answer expressed to the least number of significant figures © Copyright 1998 -2007 R. J. Rusay

Addition ð Four students were each asked to measure a piece of wire and provide a total length for the four pieces. ð Report the result correctly: 16. 346 cm

QUESTION If you were unloading a 23. 50 kg box of books from your car and a “friend” added two more 482 gram chemistry books, how much in kg and using the rules for significant digits, would you be lifting? 1. 2. 3. 4. 23. 98 kg 24. 464 kg 24. 46 kg 24. 5 kg

ANSWER Choice 3 provides both the conversion and proper number of significant digits. Consider that the 482 grams of mass must be doubled (to include both books) and that 482 grams is 0. 482 kg. When adding two measurements always report your answer to the same number of decimal places as the least precise measurement used in the calculation. In this case the answer should be reported to the hundredths place. Section 1. 5: Significant Figures and Calculations

Mathematical Processes: ð Provide correct answers assuming each value (unit omitted) is written with the correct number of sig figs: 9. 56370 9. 61370

QUESTION The average mass of a certain brand of vitamin C tablets is 253 mg. What is the mass of three such tablets rounded to the proper number of significant digits? 1. 2. 3. 4. 0. 760 grams 0. 7590 grams 0. 253 grams

ANSWER Choice 2 provides three significant digits (and accurate math work). 3 tablets 253 milligrams = 759 milligrams, then dividing by 1, 000 converts the milligrams to grams. Note three is a count of the number of objects, not a measured quantity and 759 retains the same number of significant digits as the least found in related measurements. Section 1. 5: Significant Figures and Calculations

Conversion Factor Method (Dimensional Analysis) • Qualitative Descriptions vs. Quantitative • Use exact numbers / “scale factor” UNITS • A Bookkeeping Method: Example Short” 5 ft___in 5 ___ ----> ? m • (1 ft = 12 in; 2. 54 cm = 1 in; 100 cm = 1 m) 5 5 • ___ft x 12 in/ft + ___in = 65 ___in 1. 651 65 • ___in x 2. 54 cm/in x 1 m/100 cm = ___m © Copyright 1998 -2007 R. J. Rusay