Chemistry for Dummies

Appendix A. Scientific Units: The Metric System

Much of the work chemists do involves measuring — things like the mass, volume, or length of a substance.

Because chemists must be able to communicate their measurements to other chemists all over the world, they need to speak the same measurement language. This language is the SI system of measurement (from the French Systeme International), commonly referred to as the metric system. There are actually minor differences between the SI and metric systems, but for the most part, they’re interchangeable.

The SI system is a decimal system. There are base units for mass, length, volume, and so on, and there are prefixes that modify the base units. For example, kilomeans 1,000; a kilogram is 1,000 grams, and a kilometer is 1,000 meters.

This appendix lists the SI prefixes, base units for physical quantities in the SI system, and some useful SI-English conversions.

SI Prefixes

Use Table A-l as a handy reference for the abbreviations and meanings of various SI prefixes.

Table A-1. SI (Metric) Prefixes

Length

The base unit for length in the SI system is the meter. The exact definition of meter has changed over the years, but it’s now defined as the distance that light travels in a vacuum in 1/299,792,458 of a second. Here are some SI units of length:

1 millimeter (mm) = 1,000 micrometers (μm)

1 centimeter (cm) = 10 millimeters (mm)

1 meter (m) = 100 centimeters (cm)

1 kilometer (km) = 1,000 meters (m)

Some common English to SI system length conversions are

1 mile (mi) = 1.61 kilometers (km)

1 yard (yd) = 0.914 meters (m)

1 inch (in) = 2.54 centimeters (cm)

Mass

The base unit for mass in the SI system is the kilogram. It’s the weight of the standard platinum-iridium bar found at the International Bureau of Weights and Measures. Here are some SI units of mass:

1 milligram (mg) = 1,000 micrograms (μg)

1 gram (g) = 1,000 milligrams (mg)

1 kilogram (kg) = 1,000 grams (g)

Some common English to SI system mass conversions are

1 pound (lb) = 454 grams (g)

1 ounce (oz) = 28.4 grams (g)

1 pound (lb) = 0.454 kilograms (kg)

1 grain (gr) = 0.0648 grams (g)

1 carat (car) = 200 milligrams (mg)

Volume

The base unit for volume in the SI system is the cubic meter. But chemists normally use the liter. A liter is 0.001 m³. Here are some SI units of volume:

1 milliliter (mL) = 1 cubic centimeter (cm³)

1 milliliter (mL) = 1,000 microliters (μL)

1 liter (L) = 1,000 milliliters (mL)

Some common English to SI system volume conversions are

1 quart (qt) = 0.946 liters (L)

1 pint (pt) = 0.473 liter (L)

1 fluid ounce (fl oz) = 29.6 milliliters (mL)

1 gallon (gal) = 3.78 liters (L)

Temperature

The base unit for temperature in the SI system is Kelvin. Here are the three major temperature conversion formulas:

Celsius to Fahrenheit: °F = (9/5)°C + 32

Fahrenheit to Celsius: °C = (5/9)(°F-32)

Celsius to Kelvin: °K = °C + 273

Pressure

The SI unit for pressure is the pascal, where 1 pascal equals 1 newton per square meter. But pressure can also be expressed in a number of different ways, so here are some common pressure conversions:

1 millimeter of mercury (mm Hg) = 1 torr

1 atmosphere (atm) = 760 millimeters of mercury (mm Hg) = 760 torr

1 atmosphere (atm) = 29.9 inches of mercury (in Hg)

1 atmosphere (atm) = 14.7 pounds per square inch (psi)

1 atmosphere (atm) = 101 kilopascals (kPa)

Energy

The SI unit for energy (heat being one form) is the joule, but most folks still use the metric unit of heat, the calorie. Here are some common energy conversions:

1 calorie (cal) = 4.184 joules (J)

1 food Calorie (Cal) = 1 kilocalorie (kcal) = 4,184 joules (J)

1 British thermal unit (BTU) = 252 calories (cal) = 1,053 joules (J)