Composition of Substances and Solutions, Stoichiometry, and Concentration

Vocabulary practice flashcards covering composition of substances, empirical and molecular formulas, chemical equations, stoichiometry, reaction yields, solution types, electrolytes, molarity, dilutions, and concentration units.

Percent Composition (Mass Percent)

The percentage by mass of each element in a compound, calculated as: $\frac{\text{mass of element in compound}}{\text{mass of compound}} \times 100\%$.

Empirical Formula

The simplest whole-number molar ratio of the different atoms or elements present in a chemical compound.

Molecular Formula

A representation of a molecule showing the actual number and molar ratio of atoms of each element present, which equals an integer multiple ($n$) of the empirical formula.

Formula Mass

The sum of the average atomic masses of all atoms present in a substance's formula unit, typically expressed in atomic mass units ($\text{amu}$) for ionic compounds.

Molecular Mass

The mass of a single covalent molecule in atomic mass units ($\text{amu}$), determined by adding together the average atomic masses of all atoms in the molecular formula.

Molar Mass ($\mathcal{M}$)

The mass in grams of one mole of a substance (atoms, molecules, or formula units), expressed in units of $g\,mol^{-1}$ and numerically equal to its formula or molecular mass in $\text{amu}$.

Mole ($\text{mol}$)

The SI base unit for the amount of substance, containing Avogadro's number ($6.022 \times 10^{23}$) of specified entities, which is exactly the number of atoms in $12\,g$ of carbon-12.

Avogadro's Number ($N_A$)

The fundamental constant indicating the number of constituent particles (atoms, molecules, or formula units) per mole of substance, equal to $6.022 \times 10^{23}$.

Combustion Analysis Apparatus

An analytical setup where a weighed sample containing carbon and hydrogen is combusted in excess $O_2$ inside a furnace, after which produced $H_2O$ is collected by an $Mg(ClO_4)_2$ absorber and $CO_2$ is trapped by an $NaOH$ absorber to determine elemental mass percent.

Chemical Equation

A symbolic representation that describes the proportions and atomic-level changes of reactants consumed and products formed during a chemical reaction, including their physical states ($(s)$, $(l)$, $(g)$, $(aq)$).

Combination Reaction

A chemical reaction in which two or more reactant substances combine to form a single product, such as: $SO_3(g) + H_2O(l) \rightarrow H_2SO_4(l)$.

Hydrocarbon

An organic molecular compound composed solely of hydrogen and carbon atoms that yields carbon dioxide ($CO_2$) and water ($H_2O$) upon complete combustion.

Law of Conservation of Mass

The principle stating that mass is neither created nor destroyed during a chemical reaction, meaning the total mass of the reactants must equal the total mass of the products.

Stoichiometry

The calculation and quantitative study of the mole relationships among reactants and products in a balanced chemical equation required to satisfy the conservation of mass.

Limiting Reactant

The reactant that is completely consumed first in a reaction, thereby governing and limiting the maximum theoretical amount of product that can form.

Theoretical Yield

The calculated maximum quantity of product that can be generated in a reaction assuming complete consumption of the limiting reactant according to stoichiometric ratios.

Actual Yield

The experimentally measured mass or amount of product actually obtained from a chemical reaction carried out in a laboratory.

Percent Yield

A measure of the efficiency of a chemical reaction, calculated as: $\frac{\text{Actual Yield}}{\text{Theoretical Yield}} \times 100\%$.

Solution

A homogeneous mixture of two or more substances that appears uniform in composition down to the molecular level.

Solvent

The substance in a solution that is present in the greatest quantity or greatest number of moles, which dissolves the solute.

Solute

Any substance dissolved in the solvent to produce a solution, present in a smaller amount or smaller number of moles relative to the solvent.

Aqueous Solution

A solution in which water ($H_2O$) acts as the solvent.

Concentration

A quantitative description of the relative amount of solute present in a specified volume or mass of solvent or solution.

Molarity ($M$)

The concentration unit defining the number of moles of solute dissolved per liter of solution: $M = \frac{\text{moles of solute}}{\text{liters of solution}}$.

Stock Solution

A concentrated solution with a high solute-to-solvent ratio that is routinely stored in laboratories and diluted with solvent to prepare solutions of lower concentrations.

Dilution

The process of preparing a dilute solution by adding solvent to a concentrated stock solution, keeping the total moles of solute constant according to: $M_s \times V_s = M_d \times V_d$ (or $C_1 V_1 = C_2 V_2$).

Additive Volume

The condition where the final volume ($V_2$) of a diluted mixture is calculated by summing the initial volume of the stock solution and the added volume of solvent.

Serial Dilution

A stepwise sequence of uniform dilutions where a solution is diluted repeatedly by a fixed factor, and the diluted solution ($C_2$) from one step serves as the starting stock ($C_1$) for the subsequent step.

Strong Electrolyte

A substance that dissociates nearly $100\%$ into ions when dissolved in water, allowing the resulting solution to conduct electrical current efficiently.

Weak Electrolyte

A solute that dissociates only partially into ions in aqueous solution (such as acetic acid, $CH_3COOH$), conducting electrical current weakly.

Non-electrolyte

A substance that does not ionize or form ions when dissolved in water (such as ethanol or sugar), resulting in a solution that does not conduct electric current.

Mass Percentage ($w/w\,\%$)

A concentration unit expressing the ratio of the mass of a specific solute or component to the total mass of the solution, multiplied by $100\%$.

Volume Percentage ($v/v\,\%$)

A concentration unit defined as the volume of liquid solute divided by the total volume of the solution, multiplied by $100\%$.

Parts per Million ($\text{ppm}$)

A concentration unit for very dilute solutions defined as: $\frac{\text{mass of solute}}{\text{mass of solution}} \times 10^6$, equivalent to milligrams of solute per kilogram of solvent ($mg\,kg^{-1}$).

Parts per Billion ($\text{ppb}$)

A concentration unit for trace amounts of solute defined as: $\frac{\text{mass of solute}}{\text{mass of solution}} \times 10^9$, representing one part solute per $10^9$ parts solution.