Stoichiometric Calculations and Solutions

Stoichiometric Calculations

• Steps in converting masses of reactants and products:
  • Convert the mass of substance A to moles.
  • Use the balanced chemical equation to find the mole ratio of A to B.
  • Convert the moles of substance B to mass.
• The US Space Shuttle example:
  • NASA engineers used stoichiometry to calculate fuel requirements for the space shuttles.
  • Reaction: 2H2(g) + O2(g) → 2H2O(g)
  • To calculate hydrogen needed for 1.00 tn of oxygen:
    • Convert tons of O2 to grams.
    • Calculate moles of O2.
    • Use mole ratio to find moles of H2.
    • Convert moles of H2 to tons.

Solutions

• The solubility of a solute in a solvent depends on the balance between the natural tendency for solute and solvent to mix, and achieving the lowest energy possible.
• The concentration of a solute is the amount dissolved per unit volume of solution or solvent.
• When a solute dissolves in a solvent, an equilibrium is established between the undissolved and dissolved solute.
• Polar liquids can act as solvents for ionic compounds.
• Ion-dipole interactions are strong and contribute to dissolving ionic compounds in polar liquids.
• Phospholipids form a bilayer in the cell membrane, with hydrophobic tails towards the interior and hydrophilic heads facing the exterior.

Heat of Solution

• Energy (heat) is absorbed or evolved when a solute dissolves in a solvent.
• The heat effect observed in preparing a solution is due to breaking and forming bonds and attractions between solute-solute, solvent-solvent, and solute-solvent.
• Enthalpy change (ΔH) denotes the heat change in a process between initial and final states.
• Hess's law states that the overall enthalpy change in a reaction is the sum of enthalpy changes for each step.
• Solutions can be exothermic (heat evolved) or endothermic (heat absorbed).
• Ideal solutions mix without heat change.

Molarity

• Concentration refers to the quantity of solute in a standard quantity of solution.
• A solution is dilute when the solute concentration is low and concentrated when it's high.
• Molarity (M) is the moles of solute dissolved in 1 liter of solution.
• Molarity is a measure of concentration.

Calculating Moles from Volume

• The flowchart for stoichiometric calculations can use either masses of solid reactants/products or volumes of solutions of reactants/products to determine amounts of other species.

Temperature: Random Motion of Molecules and Atoms

• Absolute Zero is -273.15 °C or -459.7 °F, where molecular motion virtually stops.
• Kelvin (K) is the same size as the Celsius degree, but with zero set at a different point.
• Temperature scales can be converted using formulas:
  • °C = (K - 273.15)
  • °F = (9/5)°C + 32
  • K = °C + 273.15

Osmosis

• Osmosis is the movement of solvent molecules through a semipermeable membrane from a dilute solution to a more concentrated solution.
• In biology, osmosis is the movement of water molecules across a cell's partially permeable membrane.
• Drinking seawater or saltwater causes dehydration due to osmosis, as water moves out of the body to try to balance the salt concentration.

Osmotic Pressure

• Osmotic pressure (π) is a colligative property of a solution that stops osmosis.
• Osmotic pressure is related to the molar concentration of solute (M) by the equation: π = MRT, where R is a constant (0.082 L.atm/K.mol) and T is the absolute temperature.