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The page below is a sample from the LabCE course Mathematic Calculations and Applications for the Histology Laboratory. Access the complete course and earn ASCLS P.A.C.E.-approved continuing education credits by subscribing online.

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Interchanging Between Percent Concentration and Molarity

Percent concentration and molarity are the most commonly used solution expressions in the histology laboratory. Interchanging these expressions involves defining percent concentration and molarity and performing ratio/proportion calculations.
What is the molarity of a 0.85% NaCl solution?
First, define the percentage solution: 0.85% w/v solution = 0.85 g/100 mL
Since molarity is defined in terms of g/L, convert 0.85 g/100 mL to g/L:
0.85 g/l00 ml = X g/1.0 L (1000 mL)
X = 0.85 x 1000 /100
X = 8.50 g/L
Once the number of grams of solute per liter is known, the molarity can be calculated:
Molarity = g of solute/GMW of solute x 1 liter
X = 8.5 g/GMW x 1.0
(GMW = Na 22.99 + Cl 35.45 = 58.44)
X = 8.5/58.44
Molarity = 0.14M