A gram is a measure of mass and is equal to 1/1,000th of a kilogram, the SI (international System) unit of mass.
A mole of a given substance is the number of grams of that substance that contain 6.022 × 1023 particles (molecules) of that substance. If this number seems arbitrary, remember it is the number of carbon atoms in exactly 12 g of carbon. Because the constituent atoms of different elements have different properties, e.g., different numbers of protons and neutrons, the number of grams in a mole of an element is unique to that element.
This number is called the molar mass or molecular weight. For carbon, as stated, it is 12. The molar masses of other elements are found in any complete periodic table of the elements, usually right under the element name or abbreviation.
The explicit relationship between grams and moles is given by:
moles of x = grams of x ÷ molar mass of x
Often, substances such as drugs are measured in micrograms, making micromoles a more convenient measure than moles. To convert from micrograms to micromoles of a substance, follow these steps:
Step 1: Look up the Molar Mass of the Substance
For example, if you have a sample of aluminum (Al), consulting the periodic table, you find that the molar mass of this element is 26.982.
Step 2: Calculate the Micrograms in the Sample
A microgram, or μg, is one one-millionth of a gram. Therefore, if you have a small 0.0062-g sample of aluminum, this equals 0.0062 × 106 = 6,200 μg.
Step 3: Convert micrograms to micromoles
Because micrograms and micromoles mathematically relate to each other in the same way grams to moles do, you can use the values on the periodic table in the same straightforward way you would in a grams-to-moles conversion.
Thus, you can convert 6,200 μg of Al to μmol of Al using the equation:
μmol of Al = 6,200 μg ÷ 26.982 μg/μmol
And find that your sample contains 229.8 μmol of aluminum.