Stoichiometry Flashcards
(8 cards)
Measuring Concentration
Concentration can be expressed by molarity (moles solute per litre solution), mass percent, mole fraction, molality, or parts per million/billion. Chemists commonly use molarity because it links substance amount directly to reaction stoichiometry. Always specify the temperature because volumes expand or contract with temperature.
Equilibrium Constant (K_eq)
The equilibrium constant K_eq is the ratio of product activities to reactant activities, each raised to the power of their stoichiometric coefficients, at a given temperature. Its magnitude locates the position of equilibrium: K≫1 means products predominate, K≪1 means reactants predominate. K_eq values are temperature‑specific.
Solubility Product (K_sp)
The solubility product K_sp is the equilibrium constant for the dissolution of a sparingly soluble ionic compound into its ions. It equals the product of the ion activities, each raised to its stoichiometric coefficient. Like all equilibrium constants, K_sp depends only on temperature.
Titration and Its Purpose
A titration is a laboratory procedure in which a solution of known concentration (the titrant) is added from a buret to a measured volume of analyte until a stoichiometric point is reached. Monitoring pH or an indicator colour change lets you detect that moment. The delivered volume then reveals the unknown concentration through simple stoichiometry.
Using M₁V₁ = M₂V₂
This dilution equation expresses conservation of moles: moles = molarity×volume, so the product of initial concentration and volume equals that of the final solution. Rearranging lets you find any one variable if the other three are known. It applies only when you dilute a single solute without additional reactions.
Calculating Molarity
Molarity M is moles of solute divided by litres of solution. Weigh the solute to obtain moles, dissolve it, then adjust the final volume in a volumetric flask. Dividing moles by that litre value yields concentration, which ties directly to stoichiometry in reactions.
Calculating K₍sp₎
Write the dissolution equation and its K₍sp₎ expression as the product of ion activities raised to coefficients. If the saturated concentration of the solid is s, plug the stoichiometric multiples (e.g., s, 2s) into that expression and compute. K₍sp₎ quantifies how far the dissolution proceeds.
pH from Concentration and K₍a₎ or K₍b₎
Start with the initial molarity of the weak acid or base, assume x moles dissociate, and write K = expression. Solve for x, which equals [H⁺] for acids or [OH⁻] for bases, then take −log to get pH. Check that x is small compared with the initial molarity to validate any approximations.
