Titration of a 50.00 ml sample of acid rain required 9.30 mL of 0.0012 M NaOH to reach the end point. What was the total [H3O+] in the rain sample? What was the pH? (see Chemistry in Action: Acid Rain).

Chemistry In Action

Acid Rain

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As the water that evaporates from oceans and lakes condenses into raindrops, it dissolves small quantities of gases from the atmosphere. Under normal conditions, rain is slightly acidic, with a pH close to 5.6, because of atmospheric CO₂ that dis- solves to form carbonic acid: COz(aq) + H2O(0) = H₂CO₂(aq) → HCO₂ (aq) + H₂0+ (aq) In recent decades, however, the acidity of rainwater in many industrialized areas of the world has increased by a factor of over 100, to a pH between 3 and 3.5. The primary cause of this so-called acid rain is industrial and automotive pollution. Each year, large power plants and smelt- ers pour millions of tons of sulfur dioxide (SO₂) gas into the atmosphere, where some is oxidized by air to produce sulfur tri- oxide (SO₂). Sulfur oxides then dissolve in rain to form dilute sulfurous acid (H₂SO₂) and sulfuric acid (H₂SO₂): SO₂(g) + H₂O(l) →→→ H₂SO₂ (aq) SO₂(g) + H₂O(1) H₂SO₂ (aq) Nitrogen oxides produced by the high-temperature reaction of N₂ with O₂ in coal-burning plants and in automobile engines fur- ther contribute to the problem. Nitrogen dioxide (NO₂) dissolves in water to form dilute nitric acid (HNO3) and nitric oxide (NO): 3 NO₂(g) + H₂O() →→→ 2 HNO₂ (aq) + NO(g) Oxides of both sulfur and nitrogen have always been pres- ent in the atmosphere, produced by such natural sources as vol- canoes and lightning bolts, but their amounts have increased dramatically over the last century because of industrialization. The result is a notable decrease in the pH of rainwater in more densely populated regions, including Europe and the eastern United States. Many processes in nature require such a fine pH balance that they are dramatically upset by the shift that has occurred in the pH of rain. Some watersheds contain soils that have high "buffering capacity" and so are able to neutralize acidic compounds in acid rain. Other areas, such as the northeastern United States and eastern Canada, where soil-buffering capac- ity is poor, have experienced negative ecological effects. Acid rain releases aluminum salts from soil, and the ions then wash into streams. The low pH and increased aluminum levels are so toxic to fish and other organisms that many lakes and streams in these areas are devoid of aquatic life. Massive tree die-offs have occurred throughout central and eastern Europe as acid rain has lowered the pH of the soil and has leached nutrients from leaves. Fortunately, adidic emissions in the United States have been greatly reduced in recent years as a result of the Clean Air Act Amendments of 1990. Industrial emissions of SO₂ and nitrogen oxides decreased by over 40% from 1990 to 2007, resulting in a decrease in acid rain depositions, particularly in the eastern United States and Canada (see accompanying figure). While significant reductions have been realized, most environmental scientists agree that additional reductions in these pollutant emissions are necessary to ensure the recovery of affected lakes and streams. This limestone statue adorning the Rheims Cathedral in France has been severely eroded by acid rain. Hydrogen ion concentration as pH from measurements made at the Central Analytical Laboratory, 1996 Sus not pr sk St HAT E suced -AP 54 .. National Deposition Program Nasional Tr 1 National Atmospheric Deposition Program National Trends Network +48 20 +47 la ** Hydrogen ion concentration as pH from measurements made at the Central Analytical Laboratory, 2009 7.0 1.4. y4 Lab ▲ These maps compare the average pH of precipitation in the United States in 1996 and in 2009. During this period, total acid deposition in much of the eastern United States decreased substantially. As the water that evaporates from oceans and lakes condenses into raindrops, it dissolves small quantities of gases from the atmosphere. Under normal conditions, rain is slightly acidic, with a pH close to 5.6, because of atmospheric CO₂ that dis- solves to form carbonic acid: COz(aq) + H2O(0) = H₂CO₂(aq) → HCO₂ (aq) + H₂0+ (aq) In recent decades, however, the acidity of rainwater in many industrialized areas of the world has increased by a factor of over 100, to a pH between 3 and 3.5. The primary cause of this so-called acid rain is industrial and automotive pollution. Each year, large power plants and smelt- ers pour millions of tons of sulfur dioxide (SO₂) gas into the atmosphere, where some is oxidized by air to produce sulfur tri- oxide (SO₂). Sulfur oxides then dissolve in rain to form dilute sulfurous acid (H₂SO₂) and sulfuric acid (H₂SO₂): SO₂(g) + H₂O(l) →→→ H₂SO₂ (aq) SO₂(g) + H₂O(1) H₂SO₂ (aq) Nitrogen oxides produced by the high-temperature reaction of N₂ with O₂ in coal-burning plants and in automobile engines fur- ther contribute to the problem. Nitrogen dioxide (NO₂) dissolves in water to form dilute nitric acid (HNO3) and nitric oxide (NO): 3 NO₂(g) + H₂O() →→→ 2 HNO₂ (aq) + NO(g) Oxides of both sulfur and nitrogen have always been pres- ent in the atmosphere, produced by such natural sources as vol- canoes and lightning bolts, but their amounts have increased dramatically over the last century because of industrialization. The result is a notable decrease in the pH of rainwater in more densely populated regions, including Europe and the eastern United States. Many processes in nature require such a fine pH balance that they are dramatically upset by the shift that has occurred in the pH of rain. Some watersheds contain soils that have high "buffering capacity" and so are able to neutralize acidic compounds in acid rain. Other areas, such as the northeastern United States and eastern Canada, where soil-buffering capac- ity is poor, have experienced negative ecological effects. Acid rain releases aluminum salts from soil, and the ions then wash into streams. The low pH and increased aluminum levels are so toxic to fish and other organisms that many lakes and streams in these areas are devoid of aquatic life. Massive tree die-offs have occurred throughout central and eastern Europe as acid rain has lowered the pH of the soil and has leached nutrients from leaves. Fortunately, adidic emissions in the United States have been greatly reduced in recent years as a result of the Clean Air Act Amendments of 1990. Industrial emissions of SO₂ and nitrogen oxides decreased by over 40% from 1990 to 2007, resulting in a decrease in acid rain depositions, particularly in the eastern United States and Canada (see accompanying figure). While significant reductions have been realized, most environmental scientists agree that additional reductions in these pollutant emissions are necessary to ensure the recovery of affected lakes and streams. This limestone statue adorning the Rheims Cathedral in France has been severely eroded by acid rain. Hydrogen ion concentration as pH from measurements made at the Central Analytical Laboratory, 1996 Sus not pr sk St HAT E suced -AP 54 .. National Deposition Program Nasional Tr 1 National Atmospheric Deposition Program National Trends Network +48 20 +47 la ** Hydrogen ion concentration as pH from measurements made at the Central Analytical Laboratory, 2009 7.0 1.4. y4 Lab ▲ These maps compare the average pH of precipitation in the United States in 1996 and in 2009. During this period, total acid deposition in much of the eastern United States decreased substantially.