The sterile solution used to rinse contact lenses can be made by dissolving 400 mg of NaCl in sterile water and diluting to 100 mL. what is the molarity of this solution?

First, find the moles of NaCl.

Because you have 400 milligrams of NaCl convert to grams by dividing 1000mg. 400mg/1000mg = 0.400 grams of NaCl.

Now we need to convert to moles. For every mole of NaCl there is 58.44 grams. (22.99g Na + 35.45 g Cl) so using stoichiometry (grams cancel) I get .0068 moles of NaCl.

Conver 100 mL to L by dividing by 1000mL. Therefore there is .001 L in the solution.

Molarity is moles per Liter. So 0.0068 mols of NaCl / 0.001 L = 6.84 M