Solutions Debriefing

Have you ever found a white residue on your pan after boiling water? Where does it come from? You may have been told this occurs because you have "hard" water. What exactly is "hard" water?

Water is said to be "hard" when metal ions (specifically Ca²⁺, Mg²⁺ and Fe²⁺) are dissolved in it. When hard water is heated, CaCO₃ is precipitated. This precipitate builds up not only on cookware, but also on pipes, eventually clogging the pipe.

Typical hard water contains around 30 mg calcium (atomic weight 40.08 g/mol) per liter of water. What is the molarity of Ca²⁺ in this solution? (You can enter your answer in scientific notation if you like. To enter 4.9 x 10^-4, type 4.9e-4)

One way to unclog the pipes is to pour hydrochloric acid down them. The acid dissolves the calcium carbonate:

Hydrochloric acid is commercially sold as muriatic acid. You can find muriatic acid at hardware stores with a concentration of 6.0 M. A hydrochloric acid solution of this concentration is very dangerous. It would be safer to use a more dilute solution. Suppose you wanted to prepare 500.0 mL of a 0.050 M hydrocholoric acid solution using the 6.0 M muriatic acid.

What volume (in mL) of muriatic acid would you need?

What volume of this solution would you need to add to 5.0 L of the hard water described above to ensure clear pipes?