UPCAT Chemistry — Stoichiometry & Chemical ReactionsCheat Sheet
Cheat sheet for UPCAT Chemistry — Stoichiometry & Chemical Reactions. Compact, printable, and organised around the concepts University of the Philippines tests most frequently in the UPCAT 2026. Perfect for the week before exam day.
Exam context
On the UPCAT 2026, the Chemistry subtest carries a "Core" weight in University of the Philippines's pattern. Stoichiometry & Chemical Reactions lands at position 4th out of 7 in the standard review order. Target score is UPG ≤ 2.2 typical, and roughly 20 items come from Chemistry on a typical UPCAT paper.
Stoichiometry & Chemical Reactions - Cheat sheet
Your last-minute revision companion for mastering stoichiometry calculations and chemical reaction types before the UPCAT exam
Sections
Formulas
Formula
1 mole = 6.022 × 10²³ particles (Avogadro's number)
Meaning
One mole contains exactly 6.022 × 10²³ atoms, molecules, or formula units
Watch Out
Don't forget to specify what type of particles (atoms, molecules, ions)
When To Use
Converting between moles and number of particles
Formula
Molar mass = Sum of atomic masses in g/mol
Meaning
Mass of one mole of a substance
Watch Out
Must use atomic masses from periodic table, not mass numbers
When To Use
Converting between grams and moles
Formula
moles = mass (g) ÷ molar mass (g/mol)
Meaning
Basic mole-mass conversion
Watch Out
Units must match - grams and g/mol
When To Use
When given mass and need to find moles
Formula
Molarity (M) = moles of solute ÷ volume of solution (L)
Meaning
Concentration in moles per litre
Watch Out
Volume must be in liters, not mL
When To Use
Solution concentration problems
Formula
M₁V₁ = M₂V₂
Meaning
Dilution formula - initial concentration × volume = final concentration × volume
Watch Out
Works only when moles of solute remain constant
When To Use
Dilution calculations
Common Values
Value
6.022 × 10²³
Symbol
Nₐ
Quantity
Avogadro's number
Value
22.4 L/mol
Symbol
Vₘ
Quantity
Molar volume at STP
Section Title
The Mole and Basic Calculations
Important Facts
- Avogadro's number = 6.022 × 10²³ particles/mol
- Atomic mass unit (u) = 1.66054 × 10⁻²⁴ g
- STP conditions: 0°C (273 K) and 1 atm pressure
- 1 mole of any gas at STP occupies 22.4 L
- Formula mass = molecular mass for covalent compounds
Key Definitions
Term
Mole
Example
1 mol of carbon = 6.022 × 10²³ carbon atoms = 12.01 g
Definition
SI unit for amount of substance; contains 6.022 × 10²³ particles
Term
Molar Mass
Example
H₂O: (2 × 1.01) + (1 × 16.00) = 18.02 g/mol
Definition
Mass of one mole of a substance in grams per mole
Term
Molecular Formula
Example
C₆H₁₂O₆ for glucose
Definition
Shows actual number and type of atoms in a molecule
Term
Empirical Formula
Example
CH₂O for glucose (C₆H₁₂O₆)
Definition
Simplest whole-number ratio of elements in a compound
Diagrams To Know
- Mole conversion triangle (mass ↔ moles ↔ particles)
- Concentration triangle (moles ↔ molarity ↔ volume)
Formulas
Formula
Reactants → Products
Meaning
Chemical equation showing transformation
Watch Out
Arrow direction matters - reactants always on left
When To Use
Representing any chemical reaction
Formula
Coefficient × Formula = Amount
Meaning
Coefficients multiply entire formulas, not just first element
Watch Out
Coefficient affects ALL atoms in the formula
When To Use
Balancing equations and stoichiometry
Section Title
Chemical Reactions and Balancing
Important Facts
- Never change subscripts when balancing - only add coefficients
- Balance one element at a time, starting with most complex compound
- Balance hydrogen and oxygen last
- Check that all atoms are balanced before finalizing
- Coefficients should be smallest whole numbers possible
Key Definitions
Term
Coefficient
Example
2H₂O means 2 water molecules
Definition
Number before formula showing how many molecules/formula units
Term
Balanced Equation
Example
2H₂ + O₂ → 2H₂O
Definition
Equal numbers of each type of atom on both sides
Term
Law of Conservation of Mass
Example
Total mass of reactants = Total mass of products
Definition
Mass cannot be created or destroyed in chemical reactions
Diagrams To Know
- Before and after particle diagrams
- Equation balancing steps
Reactions Or Equations
Note
Classic example of synthesis reaction
Equation
2H₂ + O₂ → 2H₂O
Conditions
High temperature, spark ignition
Note
Decomposition of limestone
Equation
CaCO₃ → CaO + CO₂
Conditions
High temperature heating
Section Title
Types of Chemical Reactions
Important Facts
- Single replacement requires activity series knowledge
- Double replacement often produces precipitate, gas, or water
- Combustion always involves oxygen as reactant
- Complete combustion of hydrocarbons produces only CO₂ and H₂O
- Acid-base reactions are also called neutralization reactions
Key Definitions
Term
Synthesis/Combination
Example
2Mg + O₂ → 2MgO
Definition
Two or more reactants form one product (A + B → AB)
Term
Decomposition
Example
2H₂O₂ → 2H₂O + O₂
Definition
One reactant breaks into two or more products (AB → A + B)
Term
Single Replacement
Example
Zn + CuSO₄ → ZnSO₄ + Cu
Definition
One element replaces another (A + BC → AC + B)
Term
Double Replacement
Example
AgNO₃ + NaCl → AgCl + NaNO₃
Definition
Two compounds exchange ions (AB + CD → AD + CB)
Term
Combustion
Example
CH₄ + 2O₂ → CO₂ + 2H₂O
Definition
Substance reacts with oxygen to produce CO₂ and H₂O
Term
Acid-Base
Example
HCl + NaOH → NaCl + H₂O
Definition
Acid + Base → Salt + Water
Diagrams To Know
- Reaction type identification flowchart
- Activity series chart
Reactions Or Equations
Note
Synthesis pattern
Equation
Zn + I₂ → ZnI₂
Conditions
Direct combination
Note
Decomposition pattern
Equation
2Al₂O₃ → 4Al + 3O₂
Conditions
High temperature electrolysis
Note
Single replacement pattern
Equation
Br₂ + 2KI → 2KBr + I₂
Conditions
Br₂ more active than I₂
Note
Double displacement pattern
Equation
K₂Cr₂O₇ + Ca(OH)₂ → CaCr₂O₇ + 2KOH
Conditions
Aqueous solution
Formulas
Formula
mole ratio from balanced equation
Meaning
Coefficients give mole ratios between reactants and products
Watch Out
Must use coefficients from balanced equation only
When To Use
Converting moles of one substance to moles of another
Formula
grams A → moles A → moles B → grams B
Meaning
Standard stoichiometry conversion pathway
Watch Out
Must go through moles - cannot convert mass directly
When To Use
Mass-to-mass calculations
Formula
% yield = (actual yield ÷ theoretical yield) × 100%
Meaning
Efficiency of reaction as percentage
Watch Out
Actual yield is always less than or equal to theoretical
When To Use
Comparing expected vs actual product amounts
Section Title
Stoichiometry Calculations
Important Facts
- Always start stoichiometry with balanced equation
- Identify limiting reactant by calculating moles available vs needed
- Theoretical yield is calculated from limiting reactant only
- Percent yield is never greater than 100%
- Mole ratios come from coefficients in balanced equation
Key Definitions
Term
Stoichiometry
Example
Calculating how much product forms from given reactants
Definition
Quantitative study of reactants and products in chemical reactions
Term
Limiting Reactant
Example
In 2H₂ + O₂ → 2H₂O, if only 1 mol O₂ available, it limits reaction
Definition
Reactant that is completely consumed first, limiting product formation
Term
Excess Reactant
Example
Some will remain unreacted after limiting reactant consumed
Definition
Reactant present in greater amount than needed
Term
Theoretical Yield
Example
Calculated using stoichiometry from balanced equation
Definition
Maximum amount of product possible from limiting reactant
Term
Actual Yield
Example
Usually less than theoretical due to incomplete reactions
Definition
Amount of product actually obtained from experiment
Diagrams To Know
- Stoichiometry calculation flowchart
- Limiting reactant identification steps
Formulas
Formula
% element = (mass of element in compound ÷ molar mass of compound) × 100%
Meaning
Percentage by mass of each element in compound
Watch Out
Must use mass of element times number of atoms
When To Use
Finding percent composition from molecular formula
Formula
Assume 100g sample to find empirical formula
Meaning
Percentages become grams in 100g sample
Watch Out
Must convert grams to moles, then find simplest ratio
When To Use
Converting percent composition to empirical formula
Section Title
Percent Composition and Empirical Formulas
Important Facts
- Empirical formula shows simplest ratio, molecular shows actual numbers
- To find empirical formula: % → g → mol → divide by smallest → whole numbers
- If ratios not whole numbers, multiply all by small integer
- Molecular formula = (empirical formula) × n, where n is integer
- Need molecular weight to determine n
Key Definitions
Term
Percent Composition
Example
H₂O: 11.1% H, 88.9% O
Definition
Percentage by mass of each element in a compound
Term
Empirical Formula
Example
CH₂O for glucose (actual: C₆H₁₂O₆)
Definition
Simplest whole-number ratio of atoms in compound
Term
Molecular Formula
Example
C₆H₁₂O₆ for glucose
Definition
Actual number of atoms of each element in molecule
Diagrams To Know
- Empirical formula determination flowchart
Must Remember
- Avogadro's number: 6.022 × 10²³ particles/mol
- Molar volume at STP: 22.4 L/mol for gases
- Always balance equations before stoichiometry calculations
- Limiting reactant determines theoretical yield
- Percent yield = (actual/theoretical) × 100%
- Coefficients in balanced equations give mole ratios
- Empirical formula shows simplest ratio, molecular shows actual atoms
- Molarity = moles solute/liters solution
- Never change subscripts when balancing - only coefficients
- Law of Conservation of Mass: atoms are conserved in reactions
Last Minute Tips
- For stoichiometry: always write balanced equation first, identify what's given and what's asked
- To find limiting reactant: calculate how much product each reactant can make - smallest amount wins
- When balancing equations: start with most complex molecule, save H and O for last
- For empirical formulas: assume 100g sample so percentages become grams directly
- Double-check mole calculations by ensuring units cancel properly in dimensional analysis
Comparison Tables
Rows
Values
- A + B → AB
- 2Mg + O₂ → 2MgO
- Multiple reactants, one product
Property
Synthesis
Values
- AB → A + B
- 2H₂O → 2H₂ + O₂
- One reactant, multiple products
Property
Decomposition
Values
- A + BC → AC + B
- Zn + CuSO₄ → ZnSO₄ + Cu
- More active replaces less active
Property
Single Replacement
Values
- AB + CD → AD + CB
- NaCl + AgNO₃ → NaNO₃ + AgCl
- Ion exchange, forms precipitate
Property
Double Replacement
Values
- Fuel + O₂ → CO₂ + H₂O
- CH₄ + 2O₂ → CO₂ + 2H₂O
- Always involves oxygen
Property
Combustion
Columns
- Type
- Pattern
- Example
- Key Feature
Table Title
Reaction Types Quick Reference
Rows
Values
- grams
- moles
- n = m/M
- mol = g/(g/mol)
Property
Mass to Moles
Values
- moles
- particles
- N = n × Nₐ
- particles = mol × 6.022×10²³
Property
Moles to Particles
Values
- moles
- liters (STP)
- V = n × 22.4
- L = mol × 22.4 L/mol
Property
Moles to Volume
Columns
- From
- To
- Formula
- Units
Table Title
Mole Conversion Methods
Previous chapter
Periodic Table, Bonding & Chemical Language
Next chapter
Molecular Theory — VSEPR, IMFA & KMT
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