Biochemistry Chapter on Ionization of Water

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Questions and Answers

What is the equilibrium constant (Keq) for the ionization of pure water at 25°C?

1.0 x 10^-7 M

1.8 x 10^-16 M (correct)

1.0 x 10^-14 M^2

1.0 x 10^-14 M

At 25°C, what is the concentration of hydroxide ions in pure water?

1.0 x 10^-7 M

10^-14 M

1.0 x 10^-14 M

10^-7 M (correct)

What does a pH difference of 1 indicate regarding H+ concentration?

It indicates the concentration of H+ changes by 10 times. (correct)

It means the concentrations are equal.

It means the concentration of OH- is unaffected.

It indicates the concentrations differ by 5 times.

How is pH calculated from hydrogen ion concentration?

pH = -log10([H+]) Signup and view all the answers

What is the molarity of water at 25°C?

55.5 M Signup and view all the answers

Which statement correctly describes the ionization of water?

Water undergoes slight ionization and is nearly non-conductive. Signup and view all the answers

What relationship holds true for pure water in terms of [H+] and [OH-] concentrations?

[H+] = [OH-] Signup and view all the answers

How many water molecules are ionized at equilibrium in 1L of pure water at 25°C?

2 out of every 1 billion molecules Signup and view all the answers

What is the normal range for blood plasma pH?

7.35-7.45 Signup and view all the answers

Which condition can lead to the accumulation of β-hydroxybutyric acid and acetoacetic acid?

Untreated diabetes Signup and view all the answers

Severe acidosis can result in which of the following symptoms?

Stupor and coma Signup and view all the answers

Which of the following conditions is NOT a cause of acidosis?

Excessive hydration Signup and view all the answers

What happens to enzyme functionality when the blood pH drops below the normal range?

Enzymes become less functional Signup and view all the answers

Which of the following statements accurately describes a characteristic of strong acids?

Strong acids have a greater tendency to lose protons compared to weak acids. Signup and view all the answers

What is the significance of the pKa value in relation to acid strength?

A lower pKa correlates with a stronger acid. Signup and view all the answers

In the Henderson-Hasselbalch equation, what does the ratio [A-]/[HA] represent?

The relative concentrations of the conjugate base and the weak acid. Signup and view all the answers

Which of the following correctly describes the role of a pH meter in pH determination?

It employs a glass electrode that is sensitive to H+ ions. Signup and view all the answers

What occurs during a titration to determine the concentration of an acid?

The acid reacts with a strong base, allowing calculation of its concentration. Signup and view all the answers

What is the correct expression for the acid dissociation constant (Ka)?

Ka = [H+][A-] / [HA] Signup and view all the answers

Which characteristic differentiates weak acids from strong acids?

Weak acids partially dissociate in aqueous solutions. Signup and view all the answers

How would you characterize the behavior of phenolphthalein as an indicator dye?

It indicates pH changes through a color change as protons dissociate. Signup and view all the answers

What indicates the maximal buffering power of a buffer system?

The proton donor concentration equals the proton acceptor concentration Signup and view all the answers

Which statement about the Henderson-Hasselbalch equation is true?

It can calculate pKa given pH and molar ratio of acid and base Signup and view all the answers

In a titration of acetic acid with NaOH, the resultant curve provides information about which of the following?

The pKa of the weak acid being titrated Signup and view all the answers

What characterizes the buffering action of histidine in biological systems?

It has a pKa that allows it to buffer effectively near neutral pH Signup and view all the answers

When weak acids and their conjugate bases are mixed, what is their primary function in biochemical systems?

To maintain a constant pH despite pH changes Signup and view all the answers

What happens to the ionization of histidine at neutral pH?

It is partially ionized and can act as either an acid or a base Signup and view all the answers

Which of the following defines a buffer in an aqueous system?

It resists changes in pH when significant amounts of acid or base are added Signup and view all the answers

What is the pKa value of the protonated nitrogen in the histidine side chain?

6.0 Signup and view all the answers

Study Notes

Ionization of Water

Water can ionize into hydrogen ions (H+) and hydroxide ions (OH-), a process that is reversible and slightly occurs in pure water.
Pure water is only slightly ionized, with an equilibrium constant of 1.8 x 10^-16 M at 25°C.
At equilibrium, only 2 out of every 10^9 water molecules are ionized.
The concentration of pure water at 25°C is 55.5 M.
The ion product of water (Kw) is 1 x 10^-14 M²; thus, [H+] and [OH-] are both 10^-7 M, leading to a neutral pH of 7.0.

pH Scale

pH is defined by the formula pH = -log[H+], representing the negative logarithm of hydrogen ion concentration.
A difference of 1 pH unit indicates a tenfold change in H+ concentration.
Neutral solutions, such as pure water at 25°C, have a [H+] concentration of 1.0 x 10^-7 M, corresponding to a pH of 7.0.
pH can be accurately measured using a pH meter, which employs a glass electrode sensitive to H+ concentration.

Weak Acids and Bases

Weak acids only partially dissociate in solution compared to strong acids, which completely ionize.
Acids are considered proton donors, while bases are proton acceptors; a conjugate acid-base pair consists of an acid and its corresponding base.
The Acid Dissociation Constant (Ka) indicates the strength of an acid; stronger acids have larger Ka and lower pKa values.

Henderson-Hasselbalch Equation

The equation pH = pKa + log([A-]/[HA]) relates pH, the acid's dissociation constant, and the concentrations of the acid and its conjugate base.
When pH equals pKa, the concentrations of the conjugate base and the acid are equal.

Titration of Weak Acids

Titration involves incrementally adding a strong base to an acid solution until neutralization, allowing for the determination of the acid's pKa from the resulting titration curve.
The titration curve plots pH against the volume of titrant added and reveals pKa values for weak acids.

Buffers in Biological Systems

Biological buffers consist of weak acids and their conjugate bases, maintaining a stable pH essential for biochemical processes.
Buffer effectiveness is maximized when the concentrations of the acid and base are equal.
In cells, histidine, with a pKa around 6.0, acts as a buffer near neutrality and is crucial for maintaining pH.

Acid-Base Imbalances

Normal blood plasma pH ranges from 7.35 to 7.45; deviations can lead to acidosis, affecting enzyme activity and overall metabolism.
Untreated diabetes can cause acidosis due to the accumulation of β-hydroxybutyric and acetoacetic acids, lowering blood pH significantly.
Other causes of acidosis include starvation, excessive exercise (lactic acid buildup), kidney failure, and respiratory diseases.

Enzymatic Function and pH

Enzymes have optimal pH ranges that correlate to their activity, emphasizing the importance of pH in biological reactions.

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Description

This quiz covers key concepts related to the ionization of water, weak acids, and weak electrolytes as presented in Nilsen and Cox's 'Lehninger Principles of Biochemistry'. You'll explore the slight ionization of pure water, the formation of ions, and how conductivity relates to ionization. Test your understanding of these fundamental concepts in biochemistry.