CHEM 153A Midterm 1 Exam {fall 2025/26
Update} Questions and Complete Solutions –
UCLA.
Equilibrium vs. Steady State? - correct answer *Reaction rates:
-Equilibrium: rate of reverse reaction equal to rate of forward reaction
-Steady state: rate of input into system is equal to rate of output from system
*concentrations:
- Steady State: fairly constant concentration of metabolites in the system (this is
because rate of input == rate of output)
- Equilibrium = concentrations of A and B are unchanging (because they are being
converted into each other at the same rate)
*Keq Vs. Q:
- Steady State: B/A = Q (not the same as Keq)
- Equilibrium: [B]/[A] = Keq
What's the difference between delta G & delta G naught prime? - correct answer
*delta G:
- based on conditions in the cell
- usually at steady state
- any temperature
- pH around 7.4
*delta G naught prime:
- biochemical standard state conventions
, CHEM 153A Midterm 1 Exam {fall 2025/26
Update} Questions and Complete Solutions –
UCLA.
- at equilibrium
- 25 degrees Celsius
- 1 atm
- pH 7.0
Chemical basis for pH? - correct answer - Water dissociates to form H30+ and OH-
(dissociation constant K = [H+][OH-]/[H2O])
- water is in excess, so multiply both sides by [H2O] -- this gives you K[H2O] =
[H+][OH-] = 1 *10^-4 M^2
if you take the log of [H+], you get pH
-- so basically pH is a measure of how much hydronium is present.. the lower the pH,
the higher the [H+], the more acidic the solution.. opposite is true for [OH-]
What is pKa? - correct answer - Weak acid HA and water H2O dissociate to form
H3O+ and A-
- K is equal to [H3O+][A-]/[H2O][HA], but since water is in abundance, we multiply
both sides of the equation by [H2O], which gives us K[H2O] = [H3O+][A-]/[HA]... this
is called the Ka
, CHEM 153A Midterm 1 Exam {fall 2025/26
Update} Questions and Complete Solutions –
UCLA.
- negative log Ka gives pKA... this basically tells you how much H+ and conjugate
base there are in relation to the starting weak acid
- a larger [H3O+][A-] gives a larger Ka, which means lower pKa --> lower pKa ==
stronger acid
Explain how a buffer system works. - correct answer - Buffers are solutions that can
resist changes in pH
* use weak acids
- if you add a strong base (like NaOH), it will dissociate into the ion and OH-. OH- will
react with the weak acid HA and pick up a proton, creating conjugate base A-
- if you add a strong acid (like HCl), it will dissociate into the anion and H+. the
conjugate base A- will pick up a proton and form the weak acid HA
- Thus, [H+] and [OH] concentrations don't change if you add strong acid or base
because they will both be converted into weak acid or conjugate base
Why is a buffer system important for biological systems? - correct answer - important
for proper protein function
- changes in pH affect the:
Update} Questions and Complete Solutions –
UCLA.
Equilibrium vs. Steady State? - correct answer *Reaction rates:
-Equilibrium: rate of reverse reaction equal to rate of forward reaction
-Steady state: rate of input into system is equal to rate of output from system
*concentrations:
- Steady State: fairly constant concentration of metabolites in the system (this is
because rate of input == rate of output)
- Equilibrium = concentrations of A and B are unchanging (because they are being
converted into each other at the same rate)
*Keq Vs. Q:
- Steady State: B/A = Q (not the same as Keq)
- Equilibrium: [B]/[A] = Keq
What's the difference between delta G & delta G naught prime? - correct answer
*delta G:
- based on conditions in the cell
- usually at steady state
- any temperature
- pH around 7.4
*delta G naught prime:
- biochemical standard state conventions
, CHEM 153A Midterm 1 Exam {fall 2025/26
Update} Questions and Complete Solutions –
UCLA.
- at equilibrium
- 25 degrees Celsius
- 1 atm
- pH 7.0
Chemical basis for pH? - correct answer - Water dissociates to form H30+ and OH-
(dissociation constant K = [H+][OH-]/[H2O])
- water is in excess, so multiply both sides by [H2O] -- this gives you K[H2O] =
[H+][OH-] = 1 *10^-4 M^2
if you take the log of [H+], you get pH
-- so basically pH is a measure of how much hydronium is present.. the lower the pH,
the higher the [H+], the more acidic the solution.. opposite is true for [OH-]
What is pKa? - correct answer - Weak acid HA and water H2O dissociate to form
H3O+ and A-
- K is equal to [H3O+][A-]/[H2O][HA], but since water is in abundance, we multiply
both sides of the equation by [H2O], which gives us K[H2O] = [H3O+][A-]/[HA]... this
is called the Ka
, CHEM 153A Midterm 1 Exam {fall 2025/26
Update} Questions and Complete Solutions –
UCLA.
- negative log Ka gives pKA... this basically tells you how much H+ and conjugate
base there are in relation to the starting weak acid
- a larger [H3O+][A-] gives a larger Ka, which means lower pKa --> lower pKa ==
stronger acid
Explain how a buffer system works. - correct answer - Buffers are solutions that can
resist changes in pH
* use weak acids
- if you add a strong base (like NaOH), it will dissociate into the ion and OH-. OH- will
react with the weak acid HA and pick up a proton, creating conjugate base A-
- if you add a strong acid (like HCl), it will dissociate into the anion and H+. the
conjugate base A- will pick up a proton and form the weak acid HA
- Thus, [H+] and [OH] concentrations don't change if you add strong acid or base
because they will both be converted into weak acid or conjugate base
Why is a buffer system important for biological systems? - correct answer - important
for proper protein function
- changes in pH affect the:
