Commonly Used Chemistry Metric Units

Let’s discuss the metric units that are commonly used in MCAT general chemistry:

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• Concentration, or the relative abundance of one solute in a solvent, will be measured with molarity (M), and is defined as the number of moles of a given solute per one liter of solvent.
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• Volume is typically measured in liters, a non-SI unit, but you should be familiar with the conversion factor between liters and the meter cubed (m³), the SI unit of volume, which is given by the ratio of 1,000 liters per meter cubed. Note that this also means that one milliliter (mL) is equivalent to one centimeter cubed (cm³).
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• The SI unit of pressure is the Pascal (Pa), which is defined as a Newton per meter squared. We will also commonly encounter non-SI units of pressure, particularly Torr and millimeters of mercury (mmHg), both of which are approximately equal to 133 Pa, and the atmosphere (atm, equivalent to 10⁵ Pa or 760 Torr or mmHg).
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• For temperature, we most commonly use the SI base unit Kelvin. You should also be familiar with how to convert between Kelvin and the commonly used Celsius, which is equal to the equivalent temperature in Kelvin minus 273. In equation form, this is would be: T(K) = T(°C) + 273. The Fahrenheit scale is not be used on the MCAT, and you do not need to memorize the conversion factor between Fahrenheit and Celsius.