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Ch.17 - Additional Aspects of Aqueous Equilibria
All textbooksBrown 15th EditionCh.17 - Additional Aspects of Aqueous EquilibriaProblem 112
Chapter 17, Problem 112

What is the pH at 25 C of water saturated with CO2 at a partial pressure of 1.10 atm? The Henry's law constant for CO2 at 25 C is 3.1 * 10-2 mol>L@atm.

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Step 1: Use Henry's Law to calculate the concentration of CO2 in the water. Henry's Law states that the concentration of a gas in a liquid is directly proportional to the partial pressure of the gas above the liquid. The formula is: C = kP, where C is the concentration, k is the Henry's Law constant, and P is the partial pressure.
Step 2: Substitute the given values into the Henry's Law equation. The Henry's Law constant (k) for CO2 at 25 C is 3.1 * 10^-2 mol/L*atm and the partial pressure (P) is 1.10 atm.
Step 3: The CO2 in water will form carbonic acid (H2CO3), which is a weak acid. This will dissociate into H+ and HCO3- ions. Write the chemical equation for this reaction: CO2 + H2O -> H2CO3 -> H+ + HCO3-.
Step 4: Use the concentration of CO2 calculated in step 2 as the initial concentration of H2CO3. Since we are assuming that all the CO2 forms H2CO3, the initial concentration of H+ and HCO3- is zero.
Step 5: Use the definition of pH to calculate the pH of the solution. The pH is defined as the negative logarithm (base 10) of the concentration of H+ ions in the solution. Since the concentration of H+ ions is equal to the concentration of H2CO3, you can use the concentration calculated in step 2 to find the pH.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

pH and Acidity

pH is a measure of the hydrogen ion concentration in a solution, indicating its acidity or basicity. A pH of 7 is neutral, below 7 is acidic, and above 7 is basic. The pH scale is logarithmic, meaning each whole number change represents a tenfold change in hydrogen ion concentration.
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Henry's Law

Henry's Law states that the amount of gas that dissolves in a liquid at a given temperature is proportional to the partial pressure of that gas above the liquid. This relationship is expressed as C = kH * P, where C is the concentration of the gas in the liquid, kH is the Henry's law constant, and P is the partial pressure of the gas.
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Carbonic Acid Formation

When CO2 dissolves in water, it reacts to form carbonic acid (H2CO3), which can dissociate into bicarbonate (HCO3-) and hydrogen ions (H+). This increase in hydrogen ions lowers the pH of the solution, making it more acidic. Understanding this reaction is crucial for calculating the pH of water saturated with CO2.
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Related Practice
Textbook Question

The value of Ksp for Cd(OH)2 is 2.5 × 10–14. (b) The solubility of Cd(OH)2 can be increased through formation of the complex ion CdBr42- (Kf = 5 × 103). If solid Cd(OH)2 is added to a NaBr solution, what is the initial concentration of NaBr needed to increase the molar solubility of Cd(OH)2 to 1.0 × 10-3 mol/L?

Textbook Question

(a) Write the net ionic equation for the reaction that occurs when a solution of hydrochloric acid (HCl) is mixed with a solution of sodium formate 1NaCHO22.

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The osmotic pressure of a saturated solution of strontium sulfate at 25 C is 21 torr. What is the solubility product of this salt at 25 C?

Textbook Question

A concentration of 10–100 parts per billion (by mass) of Ag+ is an effective disinfectant in swimming pools. However, if the concentration exceeds this range, the Ag+ can cause adverse health effects. One way to maintain an appropriate concentration of Ag+ is to add a slightly soluble salt to the pool. Using Ksp values from Appendix D, calculate the equilibrium concentration of Ag+ in parts per billion that would exist in equilibrium with (b) AgBr (c) AgI.