201measurements CHEM 105

MEASUREMENTS Glenn V. Lo Department of Physical Sciences Nicholls State University MEASUREMENT assigning a value to an observation For science and trade: value assigned should be in reference to or traceable to a “standard” Reported measurement generally has 2 parts: ■ Number (indicates the value) ■ Unit (indicates the standard) ■ Ex: 5.0 kg ◆ (means 5.0 times the mass of a cylinder stored in a vault at the International Bureau of Weights and Measures in France; by agreement the cylinder is defined to have a mass of 1 kg) THE STANDARDS US Trade ■ National Institute of Standards and Technology (Gaithersburg, Maryland) ■ English System of Units International/Scientific ■ International Bureau of Weights and Measures (BIPM) ■ Systeme Internationale (SI) Units ◆ Evolved from metric system The SI Units Base units for seven fundamental quantities were officially adopted in 1960 What are they? ■ mass, length, time, temperature, etc. ■ All other quantities can be derived from measurements of fundamental quantities. ■ Example: speed = length divided by time ■ Units for derived quantities are combinations of base units (speed: m/s). Some are given names (ex. Joule, J = kg m / s2) special Units of Mass SI Base unit for Mass: kg Commonly used in Chem lab: g (gram) ■ 1000 g = 1 kg, exactly ■ “k” stands for “kilo,” and means 1000 ■ k (lower case!) is an official SI abbreviation ■ 1 packet of sugar is approx. 3.5 g Relation to common English units: ■ A kilogram is a little over 2 pounds, 1 kg = 2.2 lb, ■ A pound is about ½ kilo, 1 lb = 454 g ■ An ounce is about 28 grams (8 packets of sugar) Units of Mass Commonly used unit of mass in medicines/vitamins: mg (milligram) ■ 1 mg = 0.001 g ■ “m” stands for “milli,” and means 1/1000 ■ m (lower case!) is official SI abbreviation ■ There are 1000 milligrams in a gram ■ 1000 mg = 1 g ■ One Extra Strength Tylenol has 500 mg, or ½ gram, of acetaminophen Jewelry ■ 1 carat = 200 mg Mass vs. Weight The words “weight” and “mass” are often used interchangeably, but are not the same ■ Mass is amount of matter in an object ■ Weight is the magnitude of the force that pulls an object towards the center of the earth (moon, or whatever planet it’s on) ■ Mass does not depend on location; weight does Mass is proportional to weight. In Chem. Lab.: “weigh” means “determine the mass”. Units of Length SI Base unit for Length: m (meter) Commonly used in Chem lab: cm, mm ■ 1 cm = 0.01 m, exactly (c = “centi” = 1/100) ■ 1 mm = 0.001 m, exactly (m=“milli” = 1/1000) ■ c and m (lower case!) are official SI abbreviations ■ 100 cm = 1 m, 1000 mm = 1 m, 10 mm = 1 cm Relation to common English units: ■ 1 inch is about 2½ cm, or 25 mm ◆ 1 inch = 2.54 cm = 25.4 mm, exactly ■ 1 yard is about 91 cm, about 4 a meter inches shorter than Units of Length .............................................continued......................................