MCAT General Chemistry Practice Test 7: Equilibrium

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Many caves trace their roots back to chemical principles. Some caves, called solutional caves, are formed as a result of acidic water flowing through rock such as limestone (CaCO3, Ksp = 3.4 × 10-9 at 25°C). In this process limestone is dissolved, resulting in cave formation.

Solutional caves often contain spectacular rock formations. They are the products of equilibria involving water, carbon dioxide and limestone. The process begins when surface water flowing into the caves encounters soil with a higher PCO2 than found in the atmosphere. This high CO2 content is a result of CO2 release from the earth's mantle, in a process called outgassing. Equation 1 describes the dissolution of CO2 in water:

CO2(g) + H2O(l) CO2(aq) + H2O(l)

Equation 1

Once solubilized, CO2(aq) causes the acidification of water via Equation 2:

CO2(aq) + H2O(l) H2CO3(aq) H3O+(aq) + HCO3-(aq)

Equation 2

The acidic water causes dissolution of limestone according to the following equilibrium:

Equation 3

When acidic solution finally flows through the roof and into the cave, it encounters ambient air with a lower PCO2 than that in the soil. As such, dissolved CO2(aq) is released as a gas, eventually causing the precipitation of CaCO3(s). This precipitate forms stalagmites, an upward spike formed from water striking the ground in the cave, and stalactites, which are downward spikes created from water flowing down from the roofs toward the ground. Eventually, in a process that can take thousands of years, the two can meet to form a column in the cave.

1. A local factory accidentally pollutes groundwater supplies near a cave with (NH4)2CO3. How would the cave most likely be affected?

  • A. Stalagmite and stalactite formation would likely not be affected.
  • B. Increased groundwater acidity would increase stalagmite and stalactite formation.
  • C. Increased groundwater acidity would decrease stalagmite and stalactite formation.
  • D. Cannot be determined from the information given.

2. As the ground temperature surrounding a cave increases, stalactite and stalagmite growth is found to decrease. What is the best explanation for this?

  • A. Increased heat causes CaCO3 to melt.
  • B. Formation of stalactites and stalagmites is endothermic.
  • C. Increasing temperature decreases the aqueous solubility of CaCO3.
  • D. The concentration of CO2(g) in water increases as temperature increases.

3. What is the approximate concentration of CO3-2 when enough CaCO3(s) is dissolved in water to form a saturated solution?

  • A. 2.5 × 10-9 M
  • B. 5.0 × 10-9 M
  • C. 6.0 × 10-5 M
  • D. 1.0 × 10-4 M

4. Which of the following will form a buffer when combined with CaCO3(aq)?

  • A. C2H4O2
  • B. K2CO3
  • C. NaHCO3
  • D. CO2

5. The shape of the carbonate ion is:

  • A. trigonal planar and the carbon atom is sp3 hybridized.
  • B. trigonal planar and the carbon atom is sp2 hybridized.
  • C. tetrahedral and the carbon atom is sp2 hybridized.
  • D. tetrahedral and the carbon atom is sp3 hybridized.

6. A stalagmite in a cave suffers damage and researchers want it repaired naturally as quickly as possible. Which of the following will help this process?

  • A. Adding CaCl2 to groundwater.
  • B. Adding chemical compounds in the cave that absorb CO2 from the air.
  • C. Increasing the pH of the groundwater supply.
  • D. Spraying the stalagmite with compressed CO2.