The Behavior of Substances in Water

4.19. The Behavior of Substances in Water
Part 1
a. Ammonia, NH3, is a weak electrolyte. It forms ions in solution by reacting with water molecules to form the ammonium ion and hydroxide ion. Write the balanced chemical reaction for this process, including state symbols.

b. From everyday experience you are probably aware that table sugar (sucrose), C12H22O11, is soluble in water. When sucrose dissolves in water, it doesn't form ions through any reaction with water. It just dissolves without forming ions, so it is a nonelectrolyte. Write the chemical equation for the dissolving of sucrose in water.

c. Both NH3 and C12H22O11 are soluble molecular compounds, yet they behave differently in aqueous solution. Briefly explain why one is a weak electrolyte and the other is a nonelectrolyte.

d. Hydrochloric acid, HCl, is a molecular compound that is a strong electrolyte. Write the chemical reaction of HCl with water.

e. Compare the ammonia reaction with that of hydrochloric acid. Why are both of these substances considered electrolytes?

f. Explain why HCl is a strong electrolyte and ammonia is a weak electrolyte.

g. Classify each of the following substances as either ionic or molecular.

KCl NH3 CO2 MgBr2 HCl Ca(OH)2 PbS HC2H3O2

h. For those compounds above that you classified as ionic, use the solubility rules to determine which are soluble.

i. The majority of ionic substances are solids at room temperature. Describe what you would observe if you placed a soluble ionic compound and an insoluble ionic compound in separate beakers of water.

j. Write the chemical equation(s), including state symbols, for what happens when each soluble ionic compound that you identified above is placed in water. Are these substances reacting with water when they are added to water?

k. How would you classify the soluble ionic compounds: strong electrolyte, weak electrolyte, or nonelectrolyte? Explain your answer.

l. Sodium chloride, NaCl, is a strong electrolyte, as is hydroiodic acid, HI. Write the chemical equations for what happens when these substances are added to water.

m. Are NaCl and HI strong electrolytes because they have similar behavior in aqueous solution? If not, describe, using words and equations, the different chemical process that takes place in each case.

Part 2:
You have two hypothetical molecular compounds, AX and AY. AX is a strong electrolyte and AY is a weak electrolyte. The compounds undergo the following chemical reactions when added to water.
AX(aq) + H2O(l) --> AH2O- (aq) + X-(aq)
AY(aq) + H2O(l) --> AH2O+ (aq) + Y-(aq)

a. Explain how the relative amounts of AX(aq) and AY(aq) would compare if you had a beaker of water with AX and a beaker of water with AY.

b. How would the relative amounts of X-(aq) and Y-(aq) in the two beakers compare? Be sure to explain your answer.

4.20. Working with Concentration (Molarity Concepts)
Note: You should be able to answer all of the following questions without using a calculator.
Part 1
a. Both NaCl and MgCl2 are soluble ionic compounds. Write the balanced chemical equations for these two substances dissolving in water.

b. Consider the pictures attached. 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.

c. How many moles of NaCl and MgCl2 are in each beaker?

d. How many moles of chloride ions are in each beaker? How did you arrive at this answer?

e. What is the concentration of chloride ions in each beaker? Without using mathematical equations, briefly explain how you obtained your answer.

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.
a. What would be the concentration of the MgCl2(aq) ion and of the chloride ions in the remaining solution?

b. How many moles of the MgCl2and of the chloride ions would remain in the beaker?

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 2.3 L of the 2.1M NaCl(aq) solution. You now add 1.0 L of water to this beaker.
a. What is the concentration of this NaCl(aq) solution?

b. How many moles of NaCl are present in the 1.0 L of NaCl(aq) solution?

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.