Any measurement in science must include the units associated with that measurement.
For example, if a temperature measurement were reported to be 20, would that be in Celsius, Fahrenheit, or Kelvin?
Since 273 Kelvin = 0o Celsius, imagine the confusion or even the safety risk that could result from reporting a temperature without including the units.
Suppose you give me a procedure in the lab. You tell me to raise the temperature of the reaction mixture to 303. I assume you mean Celsius, but you meant Kelvin…
I watch as the temperature rises….50…100…150…200…BOOM!
Needless to say, units are important!
Let’s start with converting temperatures. To convert from Kelvin to degrees Celsius, you’ll want to use the equation,
Degrees C = Kelvin – 273
or
Kelvin = Degrees C + 273
Notice that because Kelvin is an absolute temperature scale (it begins at 0 K, the lowest possible temperature – absolute zero, and goes up from there), we don’t use the word “degrees” Kelvin.
The Celsius scale, on the other hand, is based on the freezing and boiling points of water.
On the Celsius scale, water freezes at 0 and boils at 100, we divide the distance between into 100 equal “degrees”.
This is similar to dividing the angles of a circle into 360 equal degrees. Thus, we would say 20 degrees Celsius.
We would not say, 293 degrees Kelvin, just 293 Kelvin.
The conversion from Fahrenheit to Celsius or Celsius to Fahrenheit is a little more complicated. For those calculations, use the following equations:
EXAMPLE 1
Suppose you want to convert from 80 degrees Fahrenheit to degrees Celsius…
EXAMPLE 2
Suppose you want to convert from 45 degrees Celsius to degrees Fahrenheit…
EXAMPLE 3
Suppose you want to convert from 28 degrees Celsius to Kelvin…
EXAMPLE 4
Suppose you want to convert from 240 Kelvin to degrees Celsius…
Of course, temperature is just one of the many types of unit conversions used in chemistry. Your work in chemistry will involve everything from physical dimensions (length, volume, density) to pressure, to solution concentrations – all of which will require unit conversions at some point.
When a unit conversion does not involve temperature, your first step should be to find a definition that relates one unit to another.
EXAMPLE 5
For example, suppose you want to determine how many centimeters are in 4 inches. Begin with the relationship between cm and in, 1 in = 2.54 cm. To convert from one unit to another, write the definition as a ratio. What you want, the unit you are converting to, should be on top. What you have, the unit you are converting from, should be on bottom.
Since units work just like any other number or variable, dividing a unit by itself equals 1. By putting the unit that you already have on the bottom of the ratio, that unit will be cancelled out when you multiply the ratio by the number you are trying to convert.
How many centimeters are in 4 inches?
The metric system makes unit conversions especially simple. For this reason, it is preferred for almost any scientific calculation.
In the metric system, any measured quantity has some base unit.
For distance, the base unit is the meter.
For mass, the base unit is the gram.
For volume, the base unit is the liter.
Prefixes are used to change the base units in the metric system by some power of 10.
Here are the most commonly used prefixes in chemistry:
kilo (k) means 103
milli (m) means 10-3
micro (µ) means 10-6
nano (n) means 10-9
pico (p) means 10-12
This makes converting from one unit to another in the metric system very simple.
Where there are 36 inches in a yard and 5280 feet in a mile, there are exactly 100 cm in a meter and 1000 meters in a kilometer.
EXAMPLE 6
Try converting 0.25 L to mL. Start with the definition that relates L to mL. Rewrite it as a ratio of what you want (mL) over what you are given (L) and multiply!
EXAMPLE 7
Try converting 125684 pm to nm. Start with the definition that relates picometers (pm) to meters (m), then one that relates meters (m) to nanometers (nm). Setup the ratios so that pm and m units cancel, leaving only the desired unit of nm.
