If 15 tablespoons of NaCl are added to 16 cups of water, what is the sodium molarity of the solution?

Question

It has long been mentioned that adding a peeled potato chunk to a soup and then removing it, is a good way to remove excess salt. In addition, assume that 1.0 tablespoon equals 3.0 teaspoons; and 1.0 cup equals 0.237L.

If 15 tablespoons of NaCl are added to 16 cups of water, what is the sodium molarity of the solution? It has been determined that a specific sized potato chunk removed ½ a teaspoon amount of sodium. If 3 potato chunks were added, and removed. What is the new solution molarity of sodium?

Answer 1

OK, to determine the molarity of the sodium ions in the solution, we need to first determine some basic facts about the quantities of materials we are working with.

Molarity is defined as moles of solute (in this case, the NaCl), in the liters of final solution.

First, we are given 15 tablespoons of NaCl. First, we need to determine the mass of NaCl from this information. a typical tablespoon has the volume of approximately 14.79 cm^3, or 14.79 mL. With 15, thats a total volume of 221.85 cm^3. Now the density of NaCl solid is 2.16 g/ cm^3. Multiplying these, we get a mass of NaCl of 479.2 g of NaCl. We now need to convert the mass of NaCl to moles of NaCl, by dividing by the molar mass of 58.44 g/ mol. This gives us 8.2 moles of NaCl.

OK, now we determine the volume of water, in liters. We must convert the cups to liters. 1 cup is 0.24 liter, so multiplying by 16 we get 3.8 L of water.

Now with moles of NaCl 8.2, and liters of water 3.8, we can determine the molarity of the NaCl, which is the same as the molarity of sodium ions themselves. Dividing the moles of NaCl by the liters of water we get 2.16 Molar sodium ions in solution. We could attempt to correct the volume for the effect of the sodium chloride, but this is not easy to do without more information. The 479 g of NaCl will add some volume to the total solution, but it's not a linear effect, and in most cases, this is ignored.

Final answer is 2.16 Molar solution.