Question

Analysis of the water content of a lake found in the desert showed that it contained 17.5 percent chloride ion, and has a density of 1.23 g/mL. What is the molarity of the chloride ion in the water

Answer 1

Answer:

The molarity is 6.0 M.

Explanation:

The concentration of a solution is the amount of solute present in a given amount  of solvent, or a given amount of solution.

In this problem, we are asked to convert percent by mass to molarity, which are two different concentration units.

The percent by mass is the ratio of the mass of a solute to the mass of the solution, multiplied by 100 percent. On the other hand, molarity (M) is defined as the number of moles of solute per liter of solution.

If we have 17.15 percent chloride ion, that means that we have 17.15 grams of chloride ion in 100 grams of solution. The density provided is the density of the solution, so we calculate how many mL correspond to 100 grams of solution:

1.23 g ----------- 1 mL

100 g ------------ x= 81.3 mL

Therefore, 17.5 grams of chloride ion are contained in 81.3 mL. Now we need to convert these grams into moles.

The atomic mass of chlorine is 35.5 g/mol:

35.5 g --------------- 1 mol

17.5 g --------------- x= 0. 49 mol

This 0.49 moles are contained in 81.13 mL, the definition of molarity says that this moles are contained in a liter (or what is the same, 1000 mL):

81.3 mL --------------- 0.49 mol

1000 mL ------------- x= 6.0 M

The molarity of the chloride ion will be 6.0 M.