In the health sciences, we rarely deal with individual atoms. Instead, we deal with bulk quantities—milligrams of a drug, millimoles of an electrolyte, or grams of a nutrient. To do this, we need a way to relate the invisible mass of an atom to the visible mass we measure on a balance or scale.
The formula mass is the sum of the average atomic masses of all atoms in a substance's formula. For covalent molecules (like glucose), this is often called the molecular mass.
✏️ Question: Calculate the formula mass of Calcium Chloride (CaCl2), a common electrolyte supplement.
(Atomic masses: Ca = 40.08, Cl = 35.45)
Ca: 1 × 40.08 = 40.08
Cl: 2 × 35.45 = 70.90
Total = 110.98 amu
Because atoms are so small, we use a counting unit called the mole (mol). Just as a "dozen" always means 12, a "mole" always means 6.022 × 1023 particles. This is known as Avogadro’s Number.
This number is so large that if you had a mole of marbles, they would cover the entire Earth to a depth of several miles. However, because atoms are so tiny, a mole of water fits easily in a small medicine cup.
The Molar Mass is the mass (in grams) of one mole of a substance. It is the crucial link used to convert between "grams" (which we can weigh) and "moles" (which tells us the amount of chemical activity).
1 amu (for 1 atom) = 1 gram/mol (for 1 mole)
✏️ Question (Mass to Moles): A patient is prescribed 5.00 g of Glucose (C6H12O6). The molar mass of glucose is 180.16 g/mol. How many moles of glucose is the patient receiving?
Show AnswerUse the formula: Moles = Mass / Molar Mass
5.00 g ÷ 180.16 g/mol = 0.0278 mol
In clinical chemistry, you will move back and forth between mass, moles, and number of particles using this logic:
✏️ Question: (Moles to Particles): A sample of blood contains 0.003 moles of Sodium ions (Na+). How many individual sodium ions are in the sample?
Show AnswerUse the formula: Particles = Moles × Avogadro's Number
0.00300 mol × (6.022 × 1023) = 1.81 × 1021 ions
Blood test results often report electrolytes like Sodium or Potassium in mmol/L (millimoles per liter). This tells the physician exactly how many particles are present to interact with cell membranes, which is more clinically relevant than the total weight in milligrams. Mastering the mole allows you to understand the concentration of these vital ions.