Passage
Gastric acid is one of many acids important to biological function in the human body. Production of the main component of gastric acid, hydrochloric acid (HCl) , is accomplished in the stomach by the parietal cells, as shown in Figure 1.
Figure 1 Formation of gastric acid in the stomachFacilitated by the carbonate dehydratase enzyme (1) , the parietal cells utilize the interaction between aqueous carbon dioxide and water in the blood as a source of bicarbonate ions and acidic H⁺ ions (protons) . The H⁺ ions produced in this process are then exchanged for potassium ions through the cell membrane by a proton pump, the H⁺/K⁺-ATPase (2) . In conjunction with this proton exchange, bicarbonate ions are also exchanged for chloride ions from the interstitial fluid. As a result, H⁺ and Cl⁻ ions are concentrated in the lumen of the stomach to form aqueous hydrochloric acid.Because the production of the acid proceeds with a drop in pH from 7.0 in the cell cytosol to 1.0 in the stomach lumen, the process occurs against a concentration gradient and is endergonic, requiring the input of energy supplied by the hydrolysis of ATP. Ignoring effects from ion charge, the free energy change for ion transport across the cell membrane (ΔG_t) can be estimated by Equation 1:
∆Gt=kRT logCfCiEquation 1where R is the gas constant (8.32 J/mol·K) , k is a logarithm conversion constant, T is the absolute temperature, and C_i and C_f are the initial and final molar concentrations across the gradient, respectively.Although dilute, the gastric (hydrochloric) acid released into the lumen yields a strongly acidic environment crucial to protein digestion and the destruction of harmful micro-organisms. A comparison of its acid dissociation constant (K_a) to those of other biologically relevant organic acids is given in Table 1.Table 1 Comparison of Strengths of Selected 0.10 M Acids at 25°C
Adapted from Koolman J, Roehm KH. Color Atlas of Biochemistry. Thieme; 2011: 268-271; and Lehninger AL, Nelson DL, Cox MM. Lehninger Principles of Biochemistry. Macmillan; 2005: 397-398, 419, 520.
-Based on the biological system described in the passage, how many millimoles of hydrogen ions are present in 10.0 mL of gastric acid?

Question

Passage
Gastric acid is one of many acids important to biological function in the human body. Production of the main component of gastric acid, hydrochloric acid (HCl) , is accomplished in the stomach by the parietal cells, as shown in Figure 1.

Figure 1 Formation of gastric acid in the stomachFacilitated by the carbonate dehydratase enzyme (1) , the parietal cells utilize the interaction between aqueous carbon dioxide and water in the blood as a source of bicarbonate ions and acidic H⁺ ions (protons) . The H⁺ ions produced in this process are then exchanged for potassium ions through the cell membrane by a proton pump, the H⁺/K⁺-ATPase (2) . In conjunction with this proton exchange, bicarbonate ions are also exchanged for chloride ions from the interstitial fluid. As a result, H⁺ and Cl⁻ ions are concentrated in the lumen of the stomach to form aqueous hydrochloric acid.Because the production of the acid proceeds with a drop in pH from 7.0 in the cell cytosol to 1.0 in the stomach lumen, the process occurs against a concentration gradient and is endergonic, requiring the input of energy supplied by the hydrolysis of ATP. Ignoring effects from ion charge, the free energy change for ion transport across the cell membrane (ΔG_t) can be estimated by Equation 1:

∆Gt=kRT logCfCiEquation 1where R is the gas constant (8.32 J/mol·K) , k is a logarithm conversion constant, T is the absolute temperature, and C_i and C_f are the initial and final molar concentrations across the gradient, respectively.Although dilute, the gastric (hydrochloric) acid released into the lumen yields a strongly acidic environment crucial to protein digestion and the destruction of harmful micro-organisms. A comparison of its acid dissociation constant (K_a) to those of other biologically relevant organic acids is given in Table 1.Table 1 Comparison of Strengths of Selected 0.10 M Acids at 25°C

Adapted from Koolman J, Roehm KH. Color Atlas of Biochemistry. Thieme; 2011: 268-271; and Lehninger AL, Nelson DL, Cox MM. Lehninger Principles of Biochemistry. Macmillan; 2005: 397-398, 419, 520.
-Based on the biological system described in the passage, how many millimoles of hydrogen ions are present in 10.0 mL of gastric acid?

Quizplus · Accepted Answer

11ecba2d_11d4_8e9c_a3bf_3b8bb048036b_MD0008_00 The pH scale defines pH as the negative logarithm (base 10) of the hydrogen ion concentration (pH = −log[H⁺]). Based on this relationship, the molar concentration of hydrogen ions can be calculated as [H⁺] = 10^−pH. The molar concentration can be expressed as either moles/liter or scaled to millimoles/milliliter (mmol/mL). Multiplying a volume of the solution by its molar concentration yields the number of moles of acid solute contained in that volume of the solution. The passage states gastric acid has a pH of 1.0. Therefore, the concentration of hydrogen ions can be determined by the expression: [H⁺] = 10⁻¹ = 0.1 M = 0.1 mmol/mL The number of mmol of H⁺ in 10.0 mL of gastric acid can be determined as follows: (0.1 mmol/mL)(10.0 mL) = 1.0 mmol of H⁺ (Choice A) 100 mmol is too large by a factor of 100 and may result from algebraic errors. (Choice B) 10 mmol is too large by a factor of 10. This value may result if the pH value is mistaken as the concentration and then multiplied by the given volume of gastric acid. (Choice D) 0.1 mmol is too small by a factor of 10. This value may result by mistaking the concentration molarity (moles/liter) for moles and failing to multiply by the sample volume. Educational objective: The pH of a solution is defined as pH = −log[H⁺], where [H⁺] represents the hydrogen ion concentration in units of molarity (moles per liter). Knowing the pH of a given volume of solution allows the determination of the molar concentration and the number of moles of hydrogen ions present.

Passage Gastric Acid Is One of Many Acids Important to Biological