Molarity is another standard expression of solution concentration. Molar solutions use the gram molecular weight of a solute in calculating molar concentration in a liter (L) of solution.
The gram molecular weight (GMW) of a substance (sometimes called the "formula weight") is the sum of the combined atomic weights of all atoms in the molecule expressed in grams. For example, the GMW of NaCl is equal to the atomic weight (these atomic weights may be found on a periodic table or as a formula weight on the bottle of substance) of Na (22.99) and the atomic weight of Cl (35.45) for a total of 58.44 g.
A 1 molar (M) solution will contain 1.0 GMW of a substance dissolved in water to make 1 liter of final solution. Hence, a 1M solution of NaCl contains 58.44 g.
Example:
HCl is frequently used in enzyme histochemistry. The GMW of HCl would be the atomic weight of H added to the atomic weight of Cl: H = 1 + Cl = 35.45 = 36.45 g. A liter of 1M solution of HCl would contain 36.45 g.
Monosodium dihydrogen phosphate (NaH_{2}PO_{4}) buffer is also used in enzyme histochemistry. A liter of 1M solution of NaH_{2}PO_{4 }would contain Na (22.99) + 2 H (1x2=2) + P (30.97) + 4 O (16x4=64) for a total GMW of 119.98 g.
To simply calculate the amount or weight of a substance needed for a desired molar solution, the following formula may be used:
Weight in grams = desired molarity x volume needed in liters x GMW
-- OR --
(W = M x V x GMW)
Example:
500 mL of a 0.1M solution of NaOH is needed for a procedure. Calculate the amount of solute (NaOH) needed to prepare the solution. (atomic weights: Na = 22.99; O = 16; H = 1)
X g = 0.1M x 500 mL x GMW 39.99
X = 0.1 x 0.5 x 39.99
X = 1.9995 or 2
2.0 g of NaOH must be diluted to 500 mL to prepare a 0.1M solution.