Chapter 4, Problem 4.20QP

Working with Concentration (Molarity Concepts)

Note: You should be able to answer all of the following questions without using a calculator.

Part 1:

1. a Both NaCl and MgCl₂ are soluble ionic compounds. Write the balanced chemical equations for these two substances dissolving in water.
2. b Consider the pictures below. These pictures represent 1.0-L solutions of 1.0 M NaCl(aq) and 1.0 M MgCl₂(aq). The representations of the ions in solution are the correct relative amounts. Water molecules have been omitted for clarity. Correctly label each of the beakers, provide a key to help identify the ions, and give a brief explanation of how you made your assignments.

Keeping in mind that the pictures represent the relative amounts of ions in the solution and that the numerical information about these solutions is presented above, answer the following questions c through f.

1. c How many moles of NaCl and MgCl₂ are in each beaker?
2. d How many moles of chloride ions are in each beaker? How did you arrive at this answer?
3. e What is the concentration of chloride ions in each beaker? Without using mathematical equations, briefly explain how you obtained your answer.
4. f Explain how it is that the concentrations of chloride ions in these beakers are different even though the concentrations of each substance (compound) are the same.

Part 2: Say you were to dump out half of the MgCl₂ solution from the beaker above.

1. a What would be the concentration of the MgCl₂(aq) and of the chloride ions in the remaining solution?
2. b How many moles of the MgCl₂ and of the chloride ions would remain in the beaker?
3. c Explain why the concentration of MgCl₂(aq) would not change, whereas the number of moles of MgCl₂ would change when solution was removed from the beaker. As part of your answer, you are encouraged to use pictures.

Part 3: Consider the beaker containing 1.0 L of the 1.0 M NaCl(aq) solution. You now add 1.0 L of water to this beaker.

1. a What is the concentration of this NaCl(aq) solution?
2. b How many moles of NaCl are present in the 2.0 L of NaCl(aq) solution?
3. c Explain why the concentration of NaCl(aq) does change with the addition of water, whereas the number of moles does not change. Here again, you are encouraged to use pictures to help answer the question.