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Measurements and calculations

This chapter explores the vital role of the scientific method in conducting chemistry experiments, emphasizing how serendipity has contributed to scientific discoveries. It details the distinctions between qualitative and quantitative data and stresses the importance of controlled experiments, hypotheses, and variables. Additionally, the significance of standardized units of measurement (SI system) is covered, along with topics on mass, weight, temperature, and volume. The chapter also discusses reliability in measurements, accuracy, precision, significant digits, and common calculation methods in chemistry.

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Measurements and calculations

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  1. Chemistry Chapter 2 Measurements and calculations

  2. Scientific Method • serendipity has played a role in science • most of what we know has come by careful research and experimentation • scientific method- logical approach to solving problems by observing, collecting data, formulating hypotheses, testing hypotheses, & formulating theories supported by data

  3. quantitative data-involves numbers • measurements using rulers, thermometers, graduated cylinders, etc. • for ex- temp 25oC • qualitative data- is descriptive • for example- sulfur is a yellow chemical

  4. experiments are controlled to test one variable and collect data • system- a specific portion of matter in a given region of space is studied in an experiment or observation

  5. when scientists have a question they want answered, they usually state it in an “if-then” statement • hypothesis- testable statement (if-then)

  6. control- part of experiment that remains the same • variable- part of experiment that is changed • during the experiment, any change observed is usually due to the effects of the variable

  7. Units of Measurement • What is wrong with this recipe? Banana Nut Bread 3 flour 1 vanilla 2 eggs 2 mashed bananas 2 sugar ½ nutmeg

  8. measurements represent quantities • quantity- something that has magnitude, size, or amount (UNIT) • most ALL m’ments are a number and a unit

  9. SI System • a standard system of m’ment • 7 base units • system is monitored by International organizations • commas are NOT used in numbers = for example: 75 000 not 75,000 • (many other countries use commas as decimal points)

  10. few differences between SI system and metric • base units specific for certain quantities (table 1) • prefixes are used to indicate quantities larger or smaller than the base unit • prefixes are based on 10 (table 2)

  11. Most common prefixes • kilo– means 1000 • deci– means tenth (0.1) • centi- means hundredth (0.01) • milli- means thousandth (0.001) • commit these to memory

  12. the prefixes are used with the base units to measure larger or smaller quantities • for ex: length of room- meter distance to Sylacauga-kilometer length of book- centimeter width of fingernail- millimeter

  13. MASS • measure of the quantity of matter • base unit: SI- kilogram metric- gram • triple-beam balance

  14. Weight • measure of the force of gravity between 2 objects • can change, mass DOESN’T • SI unit - Newton • scale

  15. Time • interval between 2 occurrences • SI unit- seconds • stopwatch/clock

  16. Length • distance between 2 points • SI unit- meter • ruler

  17. Temperature • matter is composed of molecules, ions, and atoms which are in constant motion (i.e. have kinetic energy) • temp measure of the average kinetic energy of all these particles • increase heat, increase movement of particles, increase KE

  18. SI unit- Kelvin (K) • measures extreme temps • metric- Celsius (oC) • based on the freezing and boiling point of water • thermometer

  19. Derived Units • combinations of SI units • produced by multiplying or dividing std units

  20. Volume • amount of space an object takes up • SI unit- 1m3 • metric- liter (L) • 1cm3 and 1mL are smaller and usually used in the lab • 1cm3 = 1mL • graduated cylinder

  21. Volume • can be calculated using a ruler and this formula: v = l x w x h • volume relationships: 1dm3 = 1L = 1 000cm3 = 1 000mL 1 000mL = 1 000cm3

  22. Density • mass per unit volume • density = mass volume D = m v • units can be g/mL, g/cm3 (whatever units are used to measure mass and volume will be the units of density

  23. can be used to identify substances • can use the formula to find mass or volume also • density of H2O = 1g/mL

  24. How reliable are the measurements you make? • 2 important terms indicate reliability: • accuracy- how close the m’ment is to the true value • precision- how close a set of m’ments for a quantity are to each other (regardless of accuracy)

  25. % error • used to evaluate results obtained in lab • always positive number • % error =

  26. An automobile is traveling at 88 km/h. What is its speed in cm/s.

  27. Density pop quiz • A 30.0 cm3 sample of quartz has a density of 2.65g/cm3. What is the mass? • The density of a sample of cork is 0.24g/cm3. What is the volume of a 35.0g sample? • What is the density of a piece of marble with the following dimensions: 552g and 212 cm3?

  28. Significant Digits • In science, significance means measured, not importance. • the # of sig digs in a m’ment depends on the scale of instrument used • m’ment includes 1 uncertain, or estimated, digit

  29. To find sig digs: • find decimal point • find 1st non-zero digit in the sequence • that digit and everything to the right is significant • if no decimal point, count from the 1st non-zero digit to the last non-zero digit

  30. 10.0 0.002 2 000 000 25.0010 0.100 260 100 100 2.550

  31. when doing calculations on calculator, the answer cannot have any more sig digs than the value in the problem

  32. answers in addition & subtraction must contain no more digits to the right than the # with the least digit to the right in the prob 52.63 • 12.4 40.23= 40.2

  33. answer in multiplication or division must contain no more sig digs than the # with the fewest digits in the prob 18.3 x 1.4 25.62= 26 5.356 x 0.793 4.247308= 4.25

  34. Rounding Rules • # 1-4 round down 21.31 =21.3 • #6-9 round up 36.7 = 37 • # 5 -round down if # preceding 5 is even 32.5 = 32 688.5 = 688 round up if # preceding 5 is odd 43.5 = 44 759.5 = 760.

  35. 4. if there are #s after the 5, round up no matter what the preceding # is 42.52 = 43 78.571 = 79

  36. Scientific Notation • very small and very large numbers are written in this shorthand method • #s are written in this format: M x 10 n M = 1 to 9.999 n = whole number exponent

  37. convert into sci not: 650 000 000 6.5 x 108 0.000 000 974 9.74 x 10-7

  38. convert into std numbers: 3.8 x 104 38 000 1.25 x 10-3 0.001 25

  39. adding/subtracting in sci not • exponents must be same • moving decimal to LEFT increases exp • moving decimal to RIGHT decreases exp 4.5 x 105 + 3.1 x 107

  40. multiplying/dividing in sci not • multiply – ADD exponents • divide- SUBTRACT exponents 2.74 x 103 x 3.1 x 108 = 9.58 x 104 3.7 x 106

  41. Proportions: 2 types • direct proportions- if 2 quantities can be divided and you get a constant value y=kx

  42. results in a straight line as x increases, y increases

  43. 2. two quantities are inversely proportional if their product is constant xy = k

  44. forms a hyperbola • if x increases, y must decrease

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