Stoichiometry
56 flashcards covering Stoichiometry for the MCAT Chem / Phys / Psych / Soc section.
Stoichiometry is a fundamental concept in chemistry that deals with the quantitative relationships between reactants and products in chemical reactions. It helps you calculate how much of one substance is needed to react with another, or how much product will form, based on balanced chemical equations. For example, it can tell you exactly how many moles of oxygen are required to burn a certain amount of fuel, making it essential for understanding real-world processes like combustion or drug synthesis.
On the MCAT, stoichiometry appears in the Chemical and Physical Foundations section, often through questions that test your ability to perform calculations involving mole ratios, limiting reactants, and percent yield. Common traps include errors in unit conversions or failing to account for excess reagents, which can lead to incorrect answers. Focus on practicing dimensional analysis and balancing equations quickly, as these skills help tackle multi-step problems efficiently.
A concrete tip: Always verify your balanced equation before plugging in numbers.
Terms (56)
- 01
Mole
The mole is the amount of substance that contains as many entities as there are atoms in exactly 12 grams of carbon-12, equal to 6.022 × 10²³ entities.
- 02
Avogadro's number
Avogadro's number is the constant 6.022 × 10²³, representing the number of atoms, molecules, or ions in one mole of a substance.
- 03
Molar mass
Molar mass is the mass of one mole of a substance, expressed in grams per mole, and is numerically equal to the atomic or molecular weight in atomic mass units.
- 04
Balanced chemical equation
A balanced chemical equation has the same number of atoms of each element on both sides, representing the conservation of mass in a chemical reaction.
- 05
Stoichiometric coefficients
Stoichiometric coefficients are the numbers placed in front of chemical formulas in a balanced equation, indicating the mole ratios of reactants and products.
- 06
Mole ratio
The mole ratio is the ratio of moles of one substance to another in a chemical reaction, derived from the stoichiometric coefficients in the balanced equation.
- 07
Limiting reactant
The limiting reactant is the reactant that is completely consumed first in a chemical reaction, determining the maximum amount of product that can form.
- 08
Excess reactant
The excess reactant is the substance that is not completely used up in a reaction, remaining after the limiting reactant is depleted.
- 09
Theoretical yield
Theoretical yield is the maximum amount of product that can be obtained from a reaction based on the limiting reactant, calculated from stoichiometry.
- 10
Actual yield
Actual yield is the amount of product actually obtained from a reaction, which is often less than the theoretical yield due to side reactions or losses.
- 11
Percent yield
Percent yield is the ratio of actual yield to theoretical yield, multiplied by 100, indicating the efficiency of a chemical reaction.
- 12
Empirical formula
The empirical formula is the simplest whole-number ratio of atoms of each element in a compound, determined from experimental data like mass percentages.
- 13
Molecular formula
The molecular formula shows the actual number of atoms of each element in a molecule, which is a multiple of the empirical formula based on molar mass.
- 14
Percent composition
Percent composition is the percentage by mass of each element in a compound, calculated by dividing the mass of each element by the total mass and multiplying by 100.
- 15
Mass-mass stoichiometry
Mass-mass stoichiometry involves calculating the mass of one substance from the mass of another using mole ratios from a balanced equation.
- 16
Mass-volume stoichiometry
Mass-volume stoichiometry calculates the volume of a gas produced or consumed from the mass of a reactant, often using the ideal gas law.
- 17
Volume-volume stoichiometry
Volume-volume stoichiometry applies to gases at the same temperature and pressure, where volumes are proportional to mole ratios from the balanced equation.
- 18
Ideal gas law
The ideal gas law, PV = nRT, relates pressure, volume, moles, and temperature of a gas, and is used in stoichiometry for reactions involving gases.
- 19
Standard temperature and pressure (STP)
STP is defined as 0 degrees Celsius and 1 atm pressure, where one mole of an ideal gas occupies 22.4 liters, aiding in gas stoichiometry calculations.
- 20
Dalton's law of partial pressures
Dalton's law states that the total pressure of a mixture of gases is the sum of the partial pressures of the individual gases, useful in gas mixture stoichiometry.
- 21
Molarity
Molarity is the concentration of a solution defined as moles of solute per liter of solution, essential for solution stoichiometry calculations.
- 22
Solution stoichiometry
Solution stoichiometry involves calculations with reactants in solution, using molarity to determine amounts based on volumes and concentrations.
- 23
Dilution formula
The dilution formula, C1V1 = C2V2, calculates the new concentration or volume when a solution is diluted, based on conservation of moles.
- 24
Titration
Titration is a technique to determine the concentration of an unknown solution by reacting it with a known solution, using stoichiometry to reach the equivalence point.
- 25
Equivalence point
The equivalence point in a titration is when the moles of titrant added equal the moles of analyte, based on their stoichiometric ratio.
- 26
Acid-base titration
Acid-base titration neutralizes an acid with a base or vice versa, using stoichiometry to calculate concentrations from the volume at equivalence.
- 27
Combustion analysis
Combustion analysis determines the empirical formula of a compound by burning it and measuring the masses of products like CO2 and H2O.
- 28
Hydrated compound
A hydrated compound contains water molecules in its crystal structure, and stoichiometry can determine the formula by heating to remove water.
- 29
Anhydrous compound
An anhydrous compound is the form without water of crystallization, obtained after removing water from a hydrate, affecting mass calculations.
- 30
Gravimetric analysis
Gravimetric analysis determines the amount of a substance by measuring the mass of a pure compound produced in a reaction, using stoichiometry.
- 31
Strategy for identifying limiting reactant
To identify the limiting reactant, calculate the mole ratio of reactants and compare it to the stoichiometric ratio from the balanced equation.
- 32
Common trap: Assuming 1:1 ratios
A common error is assuming all reactions have 1:1 mole ratios without checking the balanced equation, leading to incorrect yield calculations.
- 33
Converting mass to moles
Converting mass to moles requires dividing the given mass by the molar mass of the substance, a fundamental step in stoichiometry problems.
- 34
Stoichiometry with density
In stoichiometry, density converts volume of a liquid to mass, as reactions typically occur based on moles, not volume directly.
- 35
Percent by mass
Percent by mass is the mass of a component divided by the total mass of the mixture, used in stoichiometry for mixtures or solutions.
- 36
Reaction stoichiometry for gases
For gases, stoichiometry often uses volumes directly if at the same T and P, or incorporates the ideal gas law for varying conditions.
- 37
Empirical formula from combustion data
To find the empirical formula from combustion, use the masses of CO2 and H2O to calculate moles of C and H, then determine the ratio with other elements.
- 38
Molecular formula determination
Determine the molecular formula by comparing the empirical formula mass to the actual molar mass obtained from experiments like vapor density.
- 39
Limiting reactant in solution
In solution reactions, the limiting reactant is found by converting volumes and concentrations to moles and comparing stoichiometric ratios.
- 40
Back titration
Back titration involves adding excess reagent and titrating the excess, using stoichiometry to calculate the original analyte amount indirectly.
- 41
Stoichiometry of redox reactions
In redox reactions, stoichiometry balances electrons transferred, ensuring the equation accounts for oxidation states and mole ratios.
- 42
Example: Calculate moles from mass
For 18 grams of water, divide by its molar mass of 18 g/mol to get 1 mole, which is then used in stoichiometric calculations.
- 43
Example: Mass of product from reactant
From 2 moles of H2 reacting with excess O2, use the mole ratio from 2H2 + O2 → 2H2O to find 2 moles of H2O, or 36 grams.
- 44
Example: Gas volume at STP
One mole of any gas at STP occupies 22.4 liters, so for 2 moles of CO2, the volume is 44.8 liters.
- 45
Example: Dilution calculation
Diluting 100 mL of 2 M HCl to 500 mL gives a new concentration of 0.4 M, using C1V1 = C2V2.
- 46
Example: Empirical formula calculation
For a compound with 40% C and 6.7% H by mass, assume 100 g, convert to moles (3.33 mol C, 6.67 mol H), and simplify to CH2.
- 47
Common trap: Unit conversion errors
Forgetting to convert grams to moles before using mole ratios can lead to incorrect stoichiometric calculations.
- 48
Strategy for percent yield problems
Calculate theoretical yield first from the limiting reactant, then divide actual yield by theoretical yield and multiply by 100 to get percent yield.
- 49
Stoichiometry in precipitation reactions
In precipitation reactions, stoichiometry predicts the mass of precipitate formed based on the limiting ion in solution.
- 50
Avogadro's hypothesis
Avogadro's hypothesis states that equal volumes of gases at the same temperature and pressure contain equal numbers of molecules, aiding gas stoichiometry.
- 51
Mole fraction
Mole fraction is the ratio of moles of a component to the total moles in a mixture, used in gas stoichiometry for partial pressures.
- 52
Titration curve interpretation
In a titration curve, the equivalence point is where pH changes sharply, determined by stoichiometric calculations of acid and base.
- 53
Stoichiometric calculations with isotopes
Isotopes affect molar mass slightly, so in precise stoichiometry, use the average atomic mass for calculations.
- 54
Reaction quotient in stoichiometry
The reaction quotient, similar to equilibrium constant but for any concentrations, helps in understanding reaction progress through stoichiometry.
- 55
Example: Limiting reactant problem
With 2 moles of A and 3 moles of B for 2A + 3B → products, A is limiting since it requires 1.5 times more B than available relative to stoichiometry.
- 56
Percent purity in stoichiometry
Percent purity accounts for impurities in a sample, so in calculations, use the actual mass of pure substance based on purity for accurate yields.