MCAT Chemistry Equations: Formula Sheet & Study Guide

MCAT Chemistry Equations: Quick Reference Sheet

Important MCAT chemistry equations include pH, pOH, the Henderson-Hasselbalch equation, Ksp, ΔG = ΔH − TΔS, q = mCΔT, the Arrhenius equation, and K_eq. These formulas appear across many MCAT chemistry concepts and high-yield MCAT topics, making them useful for solving common calculation-based questions.

As you review this sheet, focus on when each equation should be used and what information it helps you calculate, rather than simply recalling a formula.

Acid-Base Chemistry

• pH = -log[H⁺] — Acidity calculations
• pOH = -log[OH⁻] — Basicity calculations
• K_w = [H⁺][OH⁻] — Acid-base equilibrium
• pH = pK_a + log([A⁻]/[HA]) — Buffer calculations

Solutions and Solubility

• M = moles/L — Solution concentration
• M₁V₁ = M₂V₂ — Dilution calculations
• K_sp = [Cation]ᵐ[Anion]ⁿ — Solubility equilibrium

Thermodynamics

• ΔG = ΔH − TΔS — Reaction spontaneity
• q = mCΔT — Heat transfer calculations

Kinetics and Equilibrium

• k = Ae^-Ea/(RT) — Reaction rate calculations
• K_eq = [Products]/[Reactants] — Equilibrium position

Acid-Base MCAT Chemistry Equations

Acid-base questions require students to calculate pH, compare acid and base strength, and predict how buffers respond to changing conditions. The most important equations include pH, K_w, K_a, K_b, and the Henderson-Hasselbalch equation, which form the foundation of many calculation-based chemistry questions on the MCAT.

pH, pOH, and K_w

Acid-base calculations tend to begin with the relationship between hydrogen ion concentration, hydroxide ion concentration, and the ionization of water.

pH Equation

pH = -log[H₃O⁺]

Use this equation to calculate the acidity or alkalinity of a solution when a question provides a hydrogen ion concentration and asks for the pH of a solution.

K_w Equation and Finding pOH

K_w relates hydrogen ion concentration to hydroxide ion concentration. The K_w equation should be used when a question provides either hydrogen ion concentration or hydroxide ion concentration and asks you to calculate the other.

K_w = [H₃O⁺][OH^-] = 1.0 × 10^-14

Take the negative logarithm on both sides of the equation to get:

−log(K_w​​) or pK_w=−log([H₃​O⁺][OH⁻])= -log 10^-14

pK_w=pH+ pOH​ = 14

pH = pOH = 7

When a question provides pH, use this relationship to determine pOH. The same equation can be used in reverse when pOH is given.

K_a and K_b Equations

Weak acids and bases establish equilibrium in solutions, making dissociation constants important for many MCAT calculations.

K_a Equation

The equilibrium constant for a weak acid dissociation reaction is specifically known as acid dissociation constant (K_a):

K_a = [H₃O⁺][A^-]/[HA]

A larger K_a indicates a stronger acid because a greater proportion of the acid dissociates in solution.

K_b equation

The equilibrium constant for a weak base dissociation reaction is specifically known as base dissociation constant (K_b):

K_b = [BH⁺][OH^-]/[B]

Larger K_a values indicate stronger acids, while larger K_b values indicate stronger bases.

K_w equation

For conjugate acid-base pair at 25^oC:

HA + H₂O ⇌ H₃O⁺ + A^-

K_a× K_b = K_w = [H₃O]⁺[OH]^- = 10^-14

Taking negative logarithm on both sides:

pK_a​+ pK_b​=14

pK_a= -log K_a

pK_b = -logK_b

We can use these equations to determine K_b or (pK_b) of a weak base given Ka of conjugate acid. We can also calculate the K_a or(pK_a) of a weak acid given K_b of its conjugate base.

Henderson-Hasselbalch Equation

The Henderson-Hasselbalch equation is commonly used to estimate the pH of a buffer solution.

For weak acids, the equation is:

pH= pK_a+ log [A⁻] ̸ [HA]

[HA] = Concentration of weak acid

[A^-] = Concentration of Conjugate base

pK_a = acid dissociation constant of weak acid

For weak bases, the equation is:

pOH = pK_b + log[HB⁺] ̸ [B]

[HB⁺] = Concentration of conjugate acid

[B] = Concentration of weak base

pK_b = base dissociation constant of weak base

Use this equation when a question asks how a buffer's pH changes after altering the concentration of a weak acid, weak base, or conjugate pair. Increasing the ratio of conjugate base to weak acid raises pH, while decreasing the ratio lowers pH.

Quick MCAT Tip

Buffer questions test how changes in the ratio of conjugate base to weak acid affect pH. In many cases, understanding the direction of the change is more important than performing a lengthy calculation.

Common Acid-Base Calculation Mistakes on the MCAT

Many acid-base questions are missed because students select the wrong equation rather than perform the calculation incorrectly.

Common mistakes involving acid-base calculations include:

•  Forgetting that pH and pOH must add to 14
• Confusing Ka and Kb relationships for conjugate acid-base pairs
• Misidentifying conjugate acid-base pairs
• Applying the Henderson-Hasselbalch equation when a true buffer system is not present
• Missing logarithmic relationships when calculating pH

Start by identifying whether the question focuses on pH, equilibrium, or buffer chemistry based on which quantities are provided, what value must be calculated, and which equation connects them. Taking a few seconds to select the appropriate formula can help prevent avoidable errors.

Solutions and Solubility MCAT Chemistry Equations

Solutions and solubility questions commonly test concentration calculations, dilution problems, and solubility equilibria. The most important equations include molarity (M), the dilution equation

(M₁V₁ = M₂V₂), and K_sp

Molarity Equation

Concentration can be expressed using several units, including molarity, molality, mass percent, volume percent, and mole fraction. For MCAT calculations, molarity is the concentration unit tested most frequently.

M = moles of solute / liters of solution

Use M when a question provides the amount of solute and solution volume and asks you to determine concentration.

Dilution Equation

The dilution equation is used when the concentration of a solution changes while the total amount of solute remains constant.

M₁V₁ = M₂V₂

Use the dilution equation when a question asks how adding solvent affects concentration or requires you to determine the volume needed to prepare a solution of a specific concentration.

Solubility Product

The solubility product constant (K_sp) describes the equilibrium between a solid and its dissolved ions in solution.

For a general dissolution reaction:

aA(s) ⇌ cC(aq) + dD(aq)

K_sp = [C]ᶜ[D]ᵈ

Important: Pure solids are not included in the equilibrium expression.

Use K_sp when evaluating solubility equilibria or comparing the relative solubility of compounds. In general, larger K_sp values indicate greater solubility, while smaller K_sp values indicate lower solubility.

Quick MCAT Tip

Solubility questions frequently require students to compare relative solubility or predict whether a precipitate will form after solutions are mixed.

Common Solutions and Solubility Calculation Mistakes on the MCAT

Solutions and solubility questions are often missed because of calculation setup errors rather than misunderstandings of the equations themselves.

Common mistakes include:

• Confusing moles and molarity
• Forgetting to convert milliliters to liters
• Using the wrong volume in dilution calculations
• Including solids in K_sp expressions
• Ignoring stoichiometric coefficients when calculating ion concentrations

Pay close attention to whether the problem involves concentration, dilution, or solubility. These concepts often require different calculations despite using similar units. The multi-step nature of dilution and solubility calculations makes these equations worth revisiting throughout your MCAT study schedule.

Thermodynamics MCAT Chemistry Equations

Thermodynamics and kinetics equations are used to calculate heat transfer, predict reaction spontaneity, evaluate equilibrium, and analyze reaction rates. Key equations in this category include q = mCΔT, ΔG = ΔH − TΔS, ΔG° = -RTlnKeq, and the Arrhenius equation.

Calorimetry Equation

Calorimetry questions focus on calculating heat transfer during physical or chemical changes.

q = mCΔT

Where:

• q = heat energy
• m = mass
• C = specific heat capacity
• ΔT = temperature change

Apply the calorimetry equation when calculating heat absorbed or released as a substance changes temperature. Calorimetry problems often provide mass, specific heat, and temperature change and ask you to calculate heat transfer or determine one of those variables from the others.

Gibbs Free Energy Equation

Gibbs free energy predicts whether a reaction is thermodynamically favorable.

ΔG = ΔH − TΔS

Where:

• ΔH is enthalpy
•  ΔS is entropy
• T is temperature in Kelvin.

Use this equation when a question asks whether a reaction is spontaneous or how changes in enthalpy, entropy, or temperature affect reaction favorability. Negative ΔG values indicate spontaneous reactions, while positive ΔG values indicate nonspontaneous reactions.

Equilibrium and Free Energy

Free energy and equilibrium are closely related in chemical reactions.

ΔG° = -RTlnKeq

Where:

• R is the universal gas constant
• T is temperature in Kelvin
• Keq is the equilibrium constant.

This equation connects free energy and equilibrium position, so use it when a question provides either ΔG° or K_eq and asks you to determine the other value. It is also useful for evaluating whether equilibrium favors reactants or products. Larger K_eq values generally indicate that products are favored at equilibrium, while smaller K_eq values indicate that reactants are favored.

Quick MCAT Tip

Many thermodynamics questions focus on whether a process is spontaneous rather than requiring extensive calculations. Pay attention to the signs of ΔG, ΔH, and ΔS before solving the problem.

Common Thermodynamics Calculation Mistakes on the MCAT

Thermodynamics and kinetics questions are often missed when the correct concept is identified but the wrong equation is applied.

Common mistakes include:

• Confusing ΔG and ΔH
• Forgetting to convert temperature to Kelvin
• Misinterpreting positive and negative ΔG values
• Mixing up equilibrium concepts and reaction rates
• Ignoring activation energy when evaluating reaction speed

Many thermodynamics questions become easier once you distinguish between heat transfer, spontaneity, equilibrium, and reaction rate. Selecting the appropriate relationship early can simplify the problem and reduce common mistakes.

This guide focuses on MCAT chemistry equations. For formulas covering mechanics, fluids, circuits, optics, and other physics topics, explore our guide to MCAT physics equations.

Chemical Kinetics and Equilibrium Equations for the MCAT

Chemical kinetics and equilibrium questions focus on reaction rates, activation energy, reaction order, and equilibrium position. The equations below are used to determine how quickly reactions proceed and whether products or reactants are favored at equilibrium.

Reaction Rate Equation

Reaction rate describes how quickly reactants disappear or products form during a chemical reaction.

For the reaction:

A + 2B = 3C

The reaction rate can be expressed as:

-d[A]/dt = -1/2 d[B]/dt = +1/3 d[C]/dt

Where [A], [B], [C] represents the molar concentrations of the reactants and product.

Use this relationship when a question provides changes in reactant or product concentration over time and asks you to determine reaction rate or compare the rates of disappearance and formation between species.

Rate Law Equation

Rate laws relate reaction rate with the concentration or partial pressure of the reactants.

aA + bB → cC

r = k[A]^x[B]^y

Where:

• k = rate constant
• [A], [B] = molar concentration of reactants A and B
• x, y = vary for each reaction and are determined experimentally
• r = reaction rate

Questions that compare how reaction rate changes after reactant concentrations are increased, decreased, or held constant often require the rate law equation.

Reaction Order: Describes how changes in reactant concentration affect reaction rate. The overall reaction order is the sum of the exponents (x + y) and is determined experimentally, meaning it does not necessarily match the stoichiometric coefficients in a balanced chemical equation.

Rate-Determining Step: For multistep reactions, the overall reaction rate is controlled by the slowest step, known as the rate-determining step.

Arrhenius Equation

The Arrhenius equation relates reaction rate to activation energy and temperature.

k=Ae^{−Ea​/(RT)}

Where:

• A = frequency or pre-exponential factor
• e^^-Ea/(RT) = fraction of collisions that have enough energy to overcome the activation barrier
• E_a = activation energy
• R = universal gas constant
• T = temperature

Use the Arrhenius equation when a question asks how changes in temperature or activation energy affect reaction rate. Higher temperatures generally increase reaction rate by allowing more molecules to overcome the activation energy barrier.

Energy Profiles: Questions involving reaction coordinate diagrams often require students to identify activation energy before evaluating how reaction rate changes. A larger activation energy corresponds to a slower reaction under the same conditions.

Equilibrium Constant

The equilibrium constant describes the relative amounts of reactants and products present when a reaction reaches equilibrium.

aA + bB (rate forward) ⇌ cC + dD (rate backward)

k₁[A]^a[B]^b = k₂[C]^c[D]^d

Keq = [C]^c[D]^d/[A]^a[B]^b

Where:

• k₁ and k₂ are rate constants
• K_eq is the equilibrium constant

Use this equation when a question asks whether products or reactants are favored at equilibrium or how changes in concentration affect equilibrium position.

Equilibrium Shifts: MCAT questions may also apply Le Châtelier's Principle to predict how changes in concentration, pressure, or temperature affect equilibrium position.

In general:

• K_eq > 1: products are favored.
• K_eq < 1: reactants are favored.
• K_eq ≈ 1: neither side is strongly favored.

Quick MCAT Tip

Reaction rate and equilibrium position are related concepts, but they describe different aspects of a chemical reaction. A catalyst can increase reaction rate without changing the equilibrium constant.

Common Chemical Kinetics and Equilibrium Mistakes on the MCAT

Reaction speed and reaction favorability are related but distinct concepts that are frequently tested in MCAT chemistry.

Common mistakes include:

• Confusing reaction rate with equilibrium position
• Assuming a fast reaction always favors products
• Misinterpreting reaction order and stoichiometric coefficients
• Ignoring activation energy when evaluating reaction rate
• Misreading K_eq values

Separate reaction-rate concepts from equilibrium concepts, since multi-step kinetics and equilibrium questions often depend on selecting the appropriate equation before calculations begin. Because these questions combine conceptual reasoning with quantitative problem-solving, they are one reason students frequently ask how hard the MCAT is.

Biochemistry Equations You Should Know for MCAT Chemistry

Enzyme kinetics questions assess how substrate concentration, enzyme concentration, and catalytic activity influence reaction velocity. The Michaelis-Menten equation and the relationship between V_max, enzyme concentration, and k_cat are the primary enzyme kinetics equations tested on the MCAT.

Michaelis-Menten Equation

The Michaelis-Menten equation describes how reaction velocity changes as substrate concentration increases.

V₀ = (V_max x [S]) / (K_M + [S])

Where:

V₀ = reaction velocity or how fast products are being formed

•  V_max describes the maximum reaction velocity
•  [S] is the substrate concentration
•  K_m is the Michaelis constant

Questions that compare reaction velocity at different substrate concentrations often require the Michaelis-Menten equation.

K_m and V_max: K_m is the substrate concentration at which reaction velocity reaches one-half of V_max. In MCAT questions that compare enzymes, a lower Km means the enzyme reaches half-maximal activity at a lower substrate concentration.

Substrate Saturation: As substrate concentration increases, reaction velocity rises until the enzyme becomes saturated and approaches Vmax.

Michaelis-Menten Plots: Questions involving Michaelis-Menten graphs typically hold enzyme concentration constant while increasing substrate concentration.

Quick MCAT Tip

Enzyme kinetics questions often require interpreting graphs or comparing enzyme behavior after changes in substrate concentration rather than solving the equation directly.

Turnover Number and Maximum Velocity

The maximum reaction velocity of an enzyme depends on both enzyme concentration and catalytic activity.

V_max = [E] * k_cat

Where:

• V_max = maximum reaction velocity
• [E] = enzyme concentration
• k_cat = turnover number

Questions that compare enzyme activity or changes in enzyme concentration may require this relationship.

Turnover Number (k_cat): The turnover number describes how many substrate molecules a single enzyme can convert to product per unit time.

For a more in-depth breakdown of enzyme kinetics and Michaelis-Menten kinetics, refer to our guide on MCAT enzymes.

Common Biochemistry Equation Mistakes on the MCAT

Reaction velocity, substrate affinity, and maximum enzyme activity are closely related concepts that are frequently tested in enzyme kinetics questions.

Common mistakes include:

• Confusing Km with Vmax
• Assuming Vmax changes when substrate concentration changes
• Misinterpreting enzyme-substrate affinity
• Forgetting that Km occurs at one-half Vmax
• Mixing enzyme concentration with substrate concentration

Determine whether the question is testing reaction velocity, substrate affinity, enzyme concentration, or catalytic activity. Identifying which variable is changing can make enzyme kinetics easier to interpret.

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