Acid-Base and Electrolyte Balance — MCQs
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The normal value of anion gap is:
The pH of body fluids is stabilized by buffer systems. Which of the following compounds is the most effective buffer at physiologic pH?
In plasma, if pH is 5, what is the fraction of base to acid?
Most important buffer system in human blood:
Which one of the following has the maximum ionization potential?
Acid-Base and Electrolyte Balance Indian Medical PG Practice Questions and MCQs
Question 191: The normal value of anion gap is:
- A. 20-25 mEq/l
- B. 75 mEq/l
- C. 50 mEq/l
- D. 8-16 mEq/l (Correct Answer)
Explanation: ***8-16 mEq/l*** - The **anion gap** is a calculated value representing the difference between the primary measured cation (sodium) and the primary measured anions (chloride and bicarbonate) in serum. - The standard formula is: **Anion Gap = Na+ - (Cl- + HCO3-)**. - A normal anion gap typically falls within the range of **8 to 16 mEq/L**, indicating a balance between unmeasured anions and cations. *20-25 mEq/l* - An anion gap in this range would be significantly **elevated**, suggesting the presence of unmeasured anions in the blood. - This typically indicates a **high anion gap metabolic acidosis**, which can be caused by conditions like lactic acidosis or ketoacidosis. *75 mEq/l* - An anion gap of 75 mEq/L is an **extremely high** value and would be indicative of a severe life-threatening metabolic acidosis. - Such a high value would suggest a profound imbalance, likely due to a massive accumulation of **unmeasured acids**. *50 mEq/l* - An anion gap of 50 mEq/L is also significantly **elevated** and suggests a severe high anion gap metabolic acidosis. - This value is well above the normal range and would prompt immediate investigation for underlying causes of **acid accumulation**.
Question 192: The pH of body fluids is stabilized by buffer systems. Which of the following compounds is the most effective buffer at physiologic pH?
- A. NH4OH, pKa = 9.24
- B. Na2HPO4, pKa = 12.32
- C. NaH2PO4, pKa = 7.21 (Correct Answer)
- D. CH3CO2H, pKa = 4.75
Explanation: ***NaH2PO4, pKa = 7.21*** - A buffer's maximum effectiveness is typically within 1 pH unit of its **pKa value**. - With a **pKa of 7.21**, the H2PO4⁻/HPO4²⁻ buffer system (phosphate buffer) is optimally positioned to buffer fluctuations around the physiologic pH of **7.35-7.45**. - This makes the phosphate buffer system highly effective in intracellular and urinary pH regulation. *NH4OH, pKa = 9.24* - This compound is a **weak base** with a pKa of 9.24, meaning it would be effective at a pH much higher than the physiologic range. - Its buffering capacity would be minimal at **pH 7.4**, as the system would be predominantly in one form, reducing its ability to resist pH changes. *Na2HPO4, pKa = 12.32* - This represents the **second dissociation** of phosphoric acid (HPO4²⁻ ⇌ PO4³⁻ + H⁺) with a very high **pKa of 12.32**. - This dissociation occurs at extremely alkaline pH levels, far above the physiological range. - At physiologic pH, this equilibrium would be almost entirely shifted to HPO4²⁻, providing no buffering capacity. *CH3CO2H, pKa = 4.75* - **Acetic acid** has a pKa of 4.75, making it an effective buffer in the acidic range (around pH 3.75-5.75). - It would be almost entirely dissociated at **physiologic pH**, offering very little buffering capacity against pH changes in body fluids.
Question 193: In plasma, if pH is 5, what is the fraction of base to acid?
- A. 0.01
- B. 0.1 (Correct Answer)
- C. 1
- D. 10
Explanation: ***0.1*** - This question applies the **Henderson-Hasselbalch equation**: pH = pKa + log([base]/[acid]). For the **bicarbonate buffer system** (the primary plasma buffer), pKa ≈ 6.1. - Substituting the given values: $5 = 6.1 + \log([HCO_3^-] / [H_2CO_3])$ - Rearranging: $\log([HCO_3^-] / [H_2CO_3]) = 5 - 6.1 = -1.1$ - Therefore: $[HCO_3^-] / [H_2CO_3] = 10^{-1.1} ≈ 0.079$ - Among the given options, **0.079 is closest to 0.1**, making this the correct answer. - Note: pH 5 in plasma is physiologically impossible (incompatible with life), but this tests theoretical understanding of the buffer equation. *0.01* - This ratio would correspond to an even **more acidic** condition with $\log([base]/[acid]) = -2$. - Using Henderson-Hasselbalch: pH = 6.1 + (-2) = 4.1, which is lower than the given pH of 5. - The calculated ratio of 0.079 is much closer to 0.1 than to 0.01. *1* - A ratio of 1 means **equal concentrations** of base and acid, which occurs when pH = pKa. - This would give pH = 6.1, not the given pH of 5. - This represents a **neutral buffer condition**, not the acidic state described. *10* - This ratio indicates an **alkaline** solution with 10 times more base than acid. - Using Henderson-Hasselbalch: pH = 6.1 + log(10) = 6.1 + 1 = 7.1 (physiological alkalosis). - This contradicts the given acidic pH of 5.
Question 194: Most important buffer system in human blood:
- A. Hemoglobin
- B. Chloride ions
- C. Bicarbonates (Correct Answer)
- D. Phosphate buffer system
Explanation: ***Bicarbonates*** - The **bicarbonate buffer system** is the most significant extracellular buffer in human blood due to its high concentration and the ability of its components (CO2 and HCO3-) to be regulated by the lungs and kidneys, respectively. - It rapidly equilibrates with dissolved CO2, making it highly effective at buffering both acid and base imbalances to maintain **blood pH**. *Hemoglobin* - **Hemoglobin** is an important intracellular buffer within red blood cells, primarily buffering carbonic acid formed from CO2 transport. - While powerful within the red blood cell, it is not the primary buffer system in the overall plasma (extracellular fluid). *Chloride ions* - **Chloride ions** are crucial for maintaining electroneutrality in red blood cell buffering processes (e.g., the **chloride shift**), but they do not directly act as a buffer in the traditional sense of accepting or donating protons. - Their primary role in pH balance is indirect, supporting the function of other buffer systems. *Phosphate buffer system* - The **phosphate buffer system** is important, particularly in intracellular fluid and renal tubules, due to its pKa being close to physiological pH. - However, its concentration in the extracellular fluid (blood plasma) is relatively low compared to bicarbonate, making it less significant for overall blood buffering.
Question 195: Which one of the following has the maximum ionization potential?
- A. Helium ion (Correct Answer)
- B. Hydrogen ion
- C. Neutron
- D. Helium atom
Explanation: ***Helium ion (He⁺)*** - The question asks about ionization potential, which is the energy required to remove an electron from a species. A **helium ion (He⁺)** has already lost one electron, leaving only one electron bound very tightly to the nucleus with 2 protons. - The **second ionization energy of helium** (removing an electron from He⁺) is the **highest of any element** because the remaining electron experiences the full +2 nuclear charge with no shielding from other electrons. This requires approximately **54.4 eV** of energy. - This is significantly higher than the first ionization energy of any neutral atom, making He⁺ the species with the maximum ionization potential among the given options. *Hydrogen ion (H⁺)* - A **hydrogen ion (H⁺)** is a bare proton with **no electrons remaining**. Since ionization potential refers to the energy needed to remove an electron, and H⁺ has no electrons to remove, this option is technically not applicable. - However, if interpreted as asking about the hydrogen atom (H), its first ionization energy is 13.6 eV, which is much lower than the second ionization energy of helium. *Neutron* - **Neutrons** are subatomic particles with **no electric charge** and are not atoms or ions. They exist within atomic nuclei. - Since ionization potential specifically refers to removing an electron from an atom or ion, and neutrons have no electrons, they **do not have an ionization potential**. This option is not scientifically applicable to the question. *Helium atom (He)* - A neutral **helium atom** has two electrons in its 1s orbital. While helium has the **highest first ionization energy among all neutral atoms** (24.6 eV) due to its stable, filled electron shell, this is still less than half the energy required to remove an electron from He⁺ (54.4 eV). - The first ionization energy of helium is lower than the second ionization energy because removing the first electron from a neutral atom involves less electrostatic attraction than removing an electron from an already positively charged ion.
Practice by Chapter
Acid-Base Chemistry and Buffers
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pH Regulation in Body Fluids
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Respiratory Regulation of Acid-Base Balance
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Renal Regulation of Acid-Base Balance
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Respiratory and Metabolic Acidosis
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Respiratory and Metabolic Alkalosis
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Mixed Acid-Base Disorders
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Interpretation of Arterial Blood Gases
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Electrolyte Homeostasis
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Sodium and Water Balance
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Potassium Balance
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Calcium and Phosphate Metabolism
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