vA 1 molar (M) solution contains the atomic or molecular weight in grams per liter.

The molecular weight of K₂HPO₄ is 174.18 g.

Therefore, a 1M solution contains 174.18 g of K₂HPO₄ per liter.

A 1M K₂HPO₄ solution is also a 1M phosphate solution, and

it is a 2M potassium solution, because each molecule of K₂HPO₄ contains 2 potassium atoms.

The relative proportion of potassium in K₂HPO₄ is calculated from the atomic weight of potassium relative to the molecular weight:

(n × atomic weight of K) / (molecular weight of K₂HPO₄) = (2 × 39.1) / (174.18) = 0.448.

The relative proportion of phosphorus is 0.177.

Atomic weights of commonly used components in nutrient solutions:

K = 39.1

P = 30.9 (Note: The modern standard atomic weight is 30.974)

Ca = 40.08

N = 14

S = 32

Mg = 24.3

Na = 22.99

Cl = 35.45

To convert a concentration given in parts per million (ppm, e.g., commonly used for nutrient solutions) to a millimolar (mM) concentration, you divide the concentration of the element (in ppm) by the atomic weight of the element.

Example: How many mM is a solution containing 150 ppm of potassium?

150 / 39.1 = 3.83 mM