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The International System of Units

The International System of Units. 3.2. In the signs shown here, the distances are listed as numbers with no units attached. Without the units, it is impossible to communicate the measurement to others. When you make a measurement, you must assign the correct units to the numerical value. 3.2.

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The International System of Units

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  1. The International System of Units 3.2 • In the signs shown here, the distances are listed as numbers with no units attached. Without the units, it is impossible to communicate the measurement to others. When you make a measurement, you must assign the correct units to the numerical value.

  2. 3.2 Measuring with SI Units • The International System of Units (abbreviated SI), is a revised version of the metric system.

  3. 3.2 Measuring with SI Units • The five SI base units commonly used by chemists are • 1. Length - the meter (m) • 2. Mass - the kilogram (kg) • 3. Temp - the kelvin (K) • 4. Time – the second (s) • 5. Amount - the mole (mol)

  4. Volume • It is the amount of space something occupies • The SI unit of volume is the m3 • A more convenient unit of volume for everyday use is the liter (L), a non-SI unit • 1000 cm3 = 1 L • 1 mL = 1 cm3

  5. 3.2 Units and Quantities • The volume of 20 drops of liquid from a medicine dropper is approximately 1 mL.

  6. 3.2 Units and Quantities • A sugar cube has a volume of 1 cm3. 1 mL is the same as 1 cm3.

  7. 3.2 Units and Quantities • A gallon of milk has about twice the volume of a 2-L bottle of soda.

  8. Units and Quantities • Mass • A measure of the amount of matter in an object • Measured in kg (SI base unit) • A gram (g) is 1/1000 of a kilogram; the mass of 1 cm3 of water at 4°C is 1 g.

  9. Weight • a force that measures the pull on a given mass by gravity • An astronaut on the surface of the moon weighs 1/6 of what he weighs on Earth

  10. 3.2 Units and Quantities • Units of Temperature • Temperature is a measure of how hot or cold an object is. • Thermometers are used to measure temperature.

  11. 3.2 Units and Quantities • Scientists commonly use two equivalent units of temperature, the degree Celsius (˚C) and the kelvin (K).

  12. 3.2 Units and Quantities • On the Celsius scale, the freezing point of water is 0°C and the boiling point is 100°C. • On the Kelvin scale, the freezing point of water is 273 K, and the boiling point is 373K. • The zero point on the Kelvin scale, 0 K, or absolute zero, is equal to 273°C.

  13. 3.2 Units and Quantities • Because one degree on the Celsius scale is equivalent to one kelvin on the Kelvin scale, converting from one temperature to another is easy. You simply add or subtract 273, as shown in the following equations.

  14. 3.2 Units and Quantities • Conversions Between the Celsius and Kelvin Scales

  15. Sample Problems • 37˚C = ____ k • -20˚C = ____ k • 100 k = ____ ˚C • 340 k = ___ ˚C 310 253 -173 67

  16. 3.2 Section Quiz. • 1. Which of the following is not a base SI unit? • meter • gram • second • mole

  17. 3.2 Section Quiz. • 2. A temperature of 30 degrees Celsius is equivalent to • 303 K. • 300 K. • 243 K. • 247 K.

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