4. A puddle of coastal seawater, caught in a depression formed by some coastal rocks at high tide, begins to evaporate as the tide goes out. If the volume of a puddle decreases to 23% of its original volume, what is the sodium chloride concentration if it was initially 0.449 M?

0.583M NaCl

Explanation:

Molarity is an unit of concentration defined as the ratio between moles of solute and liters of solution.

In the puddle, the solute is sodium chloride that is dissolved in water and you have 0.449 moles of NaCl per liter of water

When the solution begins to evaporate, amount of water decreases whereas moles of NaCl remain constant.

As 23% of the water evaporates, amount of water that remains is 100-23 = 77%, that means now you have 0.449 moles of NaCl per 77% of a liter, 0.770L. The new concentration is:

0.449 moles NaCl / 0.770L =