22 - Water and pH in Solutions
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60 Terms
What is a solvent?
A substance that dissolves a solute to form a solution.
What is a solute?
The substance that is dissolved in a solvent.
What is a solution?
A homogeneous mixture composed of two or more substances.
Why is water called a universal solvent?
Because it dissolves many ionic and polar substances.
Why is water essential for biochemical reactions?
Cells are aqueous environments where molecules must move and interact.
What does amphiprotic mean?
A substance that can act as either an acid or a base.
How does water act as an acid?
By donating a proton (H+).
How does water act as a base?
By accepting a proton (H+).
What ions are formed when water ionizes?
Hydronium (H3O+) and hydroxide (OH−).
In pure water, what is the relationship between [H+] and [OH−]?
They are equal.
Why does water ionization equilibrium lie to the left?
Because hydronium and hydroxide ions strongly reform water.
What is Kw?
The ion product of water.
What is the value of Kw at 25°C?
1.0 × 10⁻¹⁴.
What is the concentration of H+ in pure water at 25°C?
1.0 × 10⁻⁷ M.
What is the concentration of OH− in pure water at 25°C?
1.0 × 10⁻⁷ M.
What happens to [OH−] if [H+] increases?
OH− decreases to maintain Kw.
What happens to [H+] if [OH−] increases?
H+ decreases to maintain Kw.
What does pH stand for?
Power (or potential) of hydrogen.
What is the formula for pH?
pH = −log[H+].
What is the formula for pOH?
pOH = −log[OH−].
What is the relationship between pH and pOH?
pH + pOH = 14.
What is the pKw at 25°C?
14.
What does a decrease of 1 pH unit represent?
A 10× increase in acidity.
Which has higher acidity: pH 3 or pH 5?
pH 3 (100× more acidic).
What type of solution has low pH?
Acidic solution.
What type of solution has high pH?
Basic solution.
What is Ka?
The acid dissociation constant.
What does a large Ka indicate?
A strong acid.
What is pKa?
The negative log of Ka.
What does a small pKa indicate?
A strong acid.
What is the relationship between pKa and pKb?
pKa + pKb = 14.
What is the physiological pH range important for?
Maintaining structure and function of macromolecules.
Why is pH important for enzymes?
Enzyme activity depends strongly on pH.
What defines a strong acid?
Completely ionizes in water.
What defines a weak acid?
Partially ionizes in water.
What happens when an acid loses a proton?
It becomes its conjugate base.
What is a conjugate acid-base pair?
A proton donor and its corresponding proton acceptor.
What does the Henderson–Hasselbalch equation relate?
pH to the ratio of conjugate base to acid.
What is the Henderson–Hasselbalch equation?
pH = pKa + log([A−]/[HA]).
What happens to pH when base is added to a weak acid?
pH increases.
What is titration?
Slow addition of a known solution (titrant) to an unknown solution (analyte) until neutralization.
What indicates neutralization during titration?
Often a color change.
What happens during titration of a weak acid with strong base?
The weak acid donates H+ to OH− forming water and conjugate base.
What is plotted on a titration curve x-axis?
Volume of base added.
What is plotted on a titration curve y-axis?
pH.
What is the midpoint of a titration?
Point where half the acid is neutralized
so there’s equal amounts of conjugate acid/base pairs
according to the HH equation, pH = pKa here
What is true at the midpoint of titration?
pH = pKa.
Where is buffering strongest?
Near the pKa.
What is a buffer solution?
A solution that resists changes in pH.
What determines the best buffer choice?
A pKa close to the desired pH.
What is true when [acid] = [conjugate base]?
pH = pKa.
How can a buffer be prepared?
By adjusting acid/base ratio using the Henderson–Hasselbalch equation.
What is an ampholyte?
A molecule that can act as both an acid and base.
What is an example of an ampholyte?
Glycine.
Why does glycine have two pKa values?
It has two ionizable groups.
What are glycine's ionizable groups?
Amino group and carboxyl group.
What is the charge of glycine at very low pH (~1)?
+1 (fully protonated).
What happens to glycine as pH increases from low values?
Carboxyl group loses its proton first.
What happens to glycine at high pH?
Amino group loses proton.
What is glycine's charge at very high pH?
−1 (fully deprotonated).
