An anti cavity mouthwash is 0.05% (w/v) sodium fluoride. What is the molarity of sodium fluoride in the mouthwash?

Question

How many grams of sodium fluoride are there in 0.5 fluid ounces of the mouthwash?

Answer 1

Hello Sophie

 

This question involves converting the concentration of a solution from one unit (%-w/v) to another (Molarity).

 

First, we need to determine how many grams of NaF are in 0.5 oz of the mouthwash with a concentration of 0.05%-(w/v).

 

1 oz = 29.6 mL, therefore, 0.5 oz = 14.8 mL

 

By definition, the 0.05%-(w/v) NaF solution contains 0.05 g NaF / 100 mL solution.

 

g of NaF in 0.5 oz of 0.05%-(w/v) solution = (0.05 g NaF / 100 mL solution) x 14.8 mL = 0.0074 g NaF

 

 

To calculate Molarity, we first need to convert mass of NaF from above to moles of NaF using its molar mass (42 g/mol).

 

0.0074 g NaF x 1 mol NaF / 42 g NaF = 0.00018 mol NaF

 

Finally, Molarity is moles of solute divided by the volume in units of Liters (M = mol / L)

 

Volume in Liters = 14.8 mL x 1 L / 1000 mL = 0.0148 L

 

Molarity (M) = 0.00018 mol / 0.0148 L = 0.012 M