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Essential Concepts of Measurement and Dimensional Analysis in Chemistry

This guide emphasizes the differences between qualitative and quantitative measurements, which are crucial in chemistry. It details common metric and SI units, such as liters, grams, and kelvins, and discusses the importance of using appropriate prefixes for scale. Additionally, it explores temperature conversions between Celsius and Kelvin, as well as the calculation of density and its implications in chemistry. The section on dimensional analysis highlights conversion techniques essential for problem-solving in chemistry. Mastering these concepts forms the foundation for more advanced studies.

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Essential Concepts of Measurement and Dimensional Analysis in Chemistry

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  1. Ch. 1.4 & 1.6 Notes Units of Measurement, Dimensional Analysis

  2. 1.4 • Qualitative vs. Quantitative • Qualitative – quality, subjective • Quantitative – numerical, objective • Quantitative – • In Chemistry, we use combo of metric system/SI units (Systeme International d’Unites…it’s French)

  3. 1.4 • Units • Common metric/SI units we use include: • Liters • Grams • Meters • Seconds • Kelvin • Moles

  4. 1.4 • Prefixes • We add prefixes to some of these units for larger/smaller quantities • Millimeters • Kilograms • Etc. • Table 1.5

  5. 1.4 • Why add prefixes? • 1 mm vs. 0.001 m • More efficient, appropriate for scale of measure • You don’t measure little things in meters, right?

  6. 1.4 • Temperature • Most places use ºC, U.S. uses ºF • 0ºC = freezing point of water • 100ºC = boiling point of water • Chemistry – work in ºC and K…forget all about ºF!

  7. 1.4 • Kelvin (K) is SI unit for temperature • Don’t say degrees Kelvin or ºK • Unit used most for temp. in chemistry • Most equations require temp. in K to get right answer! • 0 K is absolute zero  all motion stops, nothing colder

  8. 1.4 • K = ºC + 273.15 (must commit to memory!) • Practice! • 31.3 ºC  K • 50. K  ºC • 115 ºC  K • 393.15 K  ºC

  9. 1.4 • Answers • 304.5 K • 223 ºC • 388 K • 120.00 ºC • Did you remember sig figs? (Don’t worry, we’re going over that tomorrow!)

  10. 1.4 • Derived units • Combination of 2+ units • m/s (meters per second) • m/s (meters per second “squared”) • mol/L (moles per liter) • Etc.

  11. 1.4 • Volume • Typically use L (liters) • May see cm3 • What is this equivalent to? • Many tools for measuring volume in lab • Graduated cylinder, syringe, buret, pipet, volumetric flask/pipet

  12. 1.4 • Density = mass/volume • D = m/v • Units: usually g/mL or g/cm3 • Density of water = 1.00 g/mL • 25ºC (room temperature)

  13. 1.4 • Densities usually given at 25ºC • What happens to density if temperature… • Increases? • Decreases? • What’s an exception to this?

  14. 1.4 • Density ≠ weight • Density and water • What floats? • Sinks?

  15. 1.4 • Practice with density • Calculate the density of Hg if 1.00 x 102 g occupies a volume of 7.36 cm3. • Calculate the volume of 65.0 g of CH3OH if its density is 0.791 g/mL. • A piece of solid Au is 77.28 g and occupies a volume of 4.0 mL. What volume would a 30. g piece occupy?

  16. 1.4 • 13.6 g/cm3 • 82.2 mL • 1.6 mL

  17. 1.6 • Dimensional analysis • AKA making conversions given unit desired unit = desired unit given unit

  18. 1.6 A B = B A 500. mL .001 L = 0.500 L 1 ml

  19. 1.6 A B C = C A B 25 000 cm 1 km .01 m = 0.25000 km 1 cm 1000 m

  20. 1.6 • Dimensional analysis • Try using Mrs. B’s picket fence… at least for the last one! • 1.5 km  cm • 1000 mL  L • 13.7 m  μm (10-6) • 31 536 000 s  years

  21. 1.6 • 1.5 x 105 cm • 1 L • 13.7 x 10-6μm • 1.0000 year

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