The English System of Units
There are several systems of units, each containing units for properties such as length, volume, weight, and time. In the English system the units are defined in an arbitrary way. Length: inch (in), foot (ft), yard (yd), (yd) , mile (mi) 12 in = 1 ft 5280 ft = 1 mi 3 ft = 1 yd 1760 yd = 1 mi Volume: fluid ounce (oz), cup (c), pint (pt), quart (qt), gallon (gal) 2 c = 1 pt 32 oz = 1 qt 2 pt = 1 qt 4 qt = 1 gal Weight: ounce (oz), pound (lb), ton 16 oz = 1 lb 2000 lb = 1 ton Time: second (s), minute (min), hour (h), day (d), year (y) 60 s = 1 min 24 h = 1 d 60 min = 1 h 3651/4 d = 1 y The Metric System
The Metric System is based on the fundamental units of o f measure for length, volume, and mass. meter (m) Volume:liter Volume:liter (L) Mass : gram (g (g) Length:
Base units in the Metric System can be converted into units that are more appropriate app ropriate for the quantity being measured by adding a prefix to the name of the base unit. The common metric prefixes are given below. Metric System Prefixes Prefix femto piconanomicromillicentidecikilomegagigatera-
Symbol f p n µ m c d k M G T
Meaning x 1/1,000,000,000,000,000 (10-15) x 1/1,000,000,000,000 (10-12) x 1/1,000,000,000 (10-9) x 1/1,000,000 (10-6) x 1/1,000 (10-3) x 1/100 (10-2) x l/10(10-1) x 1,000 (103) x 1,000,000 (106) x 1,000,000,000 (109) x 1,000,000,000,000 (1012)
The base units of length and volume are linked in the metric system. By definition, a liter is equal to the volume of a cube exactly 10 cm tall, 10 cm long, and 10 cm wide. Because the volume of this cube is 1000 cubic centimeters and a liter contains 1000 milliliters, 1 milliliter is equivalent to 1 cubic centimeter. 1 mL = 1cm3 The base units of volume and weight are also linked. The gram was originally defined as the mass of 1 mL of water at 4 degrees Celsius. 1g = 1mL H2O at 4 °C Mass Versus Weight
Mass is a measure of the amount of matter in an object, so the mass of an object is constant. Weight is a measure of the force of attraction of the earth acting on an object. The weight of an object is not constant.
Mass is a more fundamental quantity than weight. There is no English equivalent to the verb weigh that can be used to describe what happens when the mass of an object is measured. You are therefore likely to encounter the terms weigh and weight for operations and quantities that are more accurately associated with the term mass. SI Units of Measure
In 1960 the International System of Units was proposed as a replacement for the Metric System. The seven base units for the SI system are given below. SI Base Units
Physical Quantity length mass time temperature electric current amount of substance luminous intensity
Name of Unit meter kilogram second kelvin ampere mole candela
Symbol m kg s K D mol cd
Derived Si Units
The units of every measurement in the SI system must be derived from one or more of the seven base units. Some of the common derived SI units used in chemistry are given below. Common Derived SI Units in Chemistry
Physical Quantity density electric charge electric potential energy force frequency pressure velocity (speed) volume
Name of Unit
Symbol kg/m3 C (A · s) V (J/C) J (kg-m2/s2) N (kg-m/s2) Hz (s-1) Pa (N/m2) m/s m3
coulomb volt joule newton hertz pascal meters per second cubic meter Non-SI Units
Strict adherence to SI units would require cha nging directions such as "add 250 mL of water to a 1-L beaker" to "add 0.00025 cubic meters of water to an 0.001-m3 container." Because of this, a number of units that are not strictly acceptable und er the SI convention are still in use. Some of these non-SI units are given below. Non-SI Units in Common Use Physical Quantity volume length pressure
energy temperature concentration
Name of Unit liter angstrom atmosphere torr electron volt degree Celsius molarity
Symbol L (10-3 m3) D (0.1 nm) atm (101.325 kPa) mmHg (133.32 Pa) eV (1.601 x 10-19 J) EC (K - 273.15) M (mol/L)
Unit Conversions
Length 1 m = 1.094 yd Volume 1 L = 1.057 qt Mass 1 g = 0.002205 lb
1 yd = 0.9144 1 qt = 0.9464 1 lb = 453.6 g
