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Introduction to Chemistry - Key Concepts and Problems

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  • Relationship between sign of ΔH_rxn and reaction type
    Negative ΔH_rxn indicates an exothermic reaction (releases heat). Positive ΔH_rxn indicates an endothermic reaction (absorbs heat).
  • Calculate moles of NH3 from 5.3 mol N2H4
    Use the balanced equation: 3 N2H4 → 4 NH3 + N2. Moles NH3 = (4/3) × moles N2H4 = (4/3) × 5.3 mol = 7.07 mol NH3.
  • Calculate moles of product from 0.112 mol reactant in 2 Ca + O2 → 2 CaO
    Moles product = moles reactant × stoichiometric ratio. For Ca: 0.112 mol Ca × (2 mol CaO / 2 mol Ca) = 0.112 mol CaO.
  • Balance Al + H2SO4 → Al2(SO4)3 + H2
    Balanced: 2 Al + 3 H2SO4 → Al2(SO4)3 + 3 H2.
  • Calculate moles of H2SO4 to react with 8.3 mol Al
    From balanced equation: 2 Al : 3 H2SO4, so moles H2SO4 = (3/2) × 8.3 mol = 12.45 mol.
  • Calculate moles of H2 formed from 0.341 mol Al
    From balanced equation: 2 Al : 3 H2SO4 : 3 H2, moles H2 = (3/2) × 0.341 mol = 0.5115 mol.
  • Calculate grams of first reactant to react with 17.3 g of second reactant in Pb + KNO3 → PbI2 + KNO3
    Use stoichiometry with molar masses and balanced equation to find required grams of first reactant.
  • Reaction of Fe with HCl: Fe + 2 HCl → FeCl2 + H2
    Calculate minimum mass of HCl to dissolve 2.8 g Fe and moles of H2 produced using molar masses and stoichiometry.
  • Calculate theoretical yield of TiCl4 from Ti and Cl2
    Balanced: Ti + 2 Cl2 → TiCl4. Theoretical yield depends on limiting reactant from initial moles.
  • Calculate percent yield from theoretical 0.118 g and actual 0.104 g
    Percent yield = (actual yield / theoretical yield) × 100 = (0.104 / 0.118) × 100 ≈ 88.14%.
  • Limiting reactant and percent yield in 4 HCl + O2 → 2 H2O + 2 Cl2
    Determine limiting reactant by mole comparison; calculate theoretical Cl2 yield and percent yield from actual Cl2 collected.
  • Classify melting ice, sparkler burning, acetone evaporating by ΔH_rxn sign
    Ice melting: endothermic (+ΔH). Sparkler burning: exothermic (-ΔH). Acetone evaporating: endothermic (+ΔH).
  • Heat produced by combustion of 237 g CH4, ΔH_rxn = -802.3 kJ
    Calculate moles CH4, multiply by ΔH_rxn per mole to find total heat released.
  • Dissolving calcium carbonate with HCl in toilet cleaner
    Write balanced reaction; calculate grams CaCO3 dissolved by 5.8 g HCl using stoichiometry.
  • CO2 produced from combustion of 18.9 L propane (density 0.621 g/mL)
    Convert volume to mass, write balanced combustion equation, calculate moles propane and CO2 produced.
  • Combustion of ethanol: find limiting reactant, theoretical H2O, percent yield
    Use volumes and densities to find moles reactants and products; calculate limiting reactant and yields.
  • Bohr model electron transition
    Electron moves between orbits by absorbing or emitting energy equal to the difference in energy levels.
  • Electron configuration for N, Mg, Ar, Se
    Write full configurations using Aufbau principle for each element.
  • Electron configuration using noble gas core for Te, Br, I, Cs
    Use previous noble gas in brackets followed by valence electron configuration.
  • Number of valence electrons in Ba, Al, Be, Se
    Ba: 2, Al: 3, Be: 2, Se: 6 valence electrons.
  • Orbital diagrams and unpaired electrons for Ne, I, Sr, Ge
    Draw orbital boxes for valence electrons; count unpaired electrons.
  • Number of 3d electrons in Fe, Zn, K, As
    Fe: 6, Zn: 10, K: 0, As: 10 electrons in 3d subshell.
  • Identify element from electron configuration: Ne]3s1, [Kr]5s24d10, [Xe]6s2, [Kr]5s24d105p3
    Match configurations to elements based on valence electrons and subshell filling.
  • Element with higher ionization energy: Al or In, Cl or Sb, K or Ge, S or Se
    Higher ionization energy: Al, Cl, Ge, S.
  • Order elements by increasing ionization energy: Ga, In, F, Si, N
    Order: In < Ga < Si < N < F.
  • Element with larger atoms: Sn or Si, Br or Ga, Sn or Bi, Se or Sn
    Larger atoms: Sn, Ga, Bi, Sn.
  • Order elements by increasing atomic size: Cs, Sb, S, P, Se
    Order: S < P < Se < Sb < Cs.
  • More metallic element from pairs: Sb or Pb, K or Ge, Ge or Sb, As or Sn
    More metallic: Pb, K, Sb, Sn.
  • Electron configuration for ions F-, P3-, Li+, Al3+
    All have electron configuration of nearest noble gas: Ne or Ar.
  • Electron configuration and Lewis structure for Li, P, F, Ne
    Show valence electrons and indicate electrons included in Lewis structure.
  • Generic Lewis structure for alkali metals and electron gain/loss
    Alkali metals lose 1 electron to form +1 ions in chemical reactions.
  • Lewis structures for ions Sr2+, S2-, Li+, Cl-
    Show full octet for anions and empty valence for cations.
  • Noble gas with same Lewis structure as Se2-, I-, Sr2+, F-
    Se2-, I-, F- resemble Ar; Sr2+ resembles Kr.
  • Lewis structure for ionic compounds from Al and F, O, N
    Show Al3+ ion with corresponding anions F-, O2-, N3- with full octets.
  • Lewis structures for CH4, NF3, OF2, H2O
    Draw structures showing bonding and lone pairs for each molecule.
  • Lewis structures with resonance for ClO3-, ClO4-, NO3-, SO3
    Include resonance forms with delocalized electrons for these ions.
  • Number of electron groups around central atom in CH2Cl2, SBr2, H2S, PCl3
    Count bonding and lone pairs: CH2Cl2=4, SBr2=4, H2S=4, PCl3=4.
  • Order elements by increasing electronegativity: Ba, N, F, Si, Cs
    Order: Cs < Ba < Si < N < F.
  • Electron and molecular geometries of CH3OH, H3COCH3, H2O2
    CH3OH: tetrahedral; H3COCH3: tetrahedral around each C; H2O2: bent.
  • Classify I2, NO, HCl, N2 as polar or nonpolar
    I2 and N2 are nonpolar; NO and HCl are polar molecules.
  • Percent composition and molecular formula of diazomethane
    Use mass percentages and molar mass to find empirical and molecular formula; draw Lewis structure.
  • Lewis dot structure, geometry, polarity, and classification for CS2, NCl3, CF2, CH2F2
    Draw Lewis structures; determine molecular geometry; indicate bond polarity; classify as polar or nonpolar.
  • Convert 921 torr, 4.8 × 10^4 Pa, 87.5 psi, 34.22 in Hg to atm
    Use conversion factors: 1 atm = 760 torr = 101325 Pa = 14.7 psi = 29.92 in Hg.
  • Calculate pressure after compressing gas from 22.8 L at 1.65 atm to 10.7 L
    Use Boyle's law: P1V1 = P2V2; solve for P2.
  • Volume change of balloon cooling from 298 K to 77 K
    Use Charles's law: V1/T1 = V2/T2; calculate new volume at 77 K.
  • Final volume of oxygen gas cooled from 95.3 °C to 0.0 °C at constant pressure
    Use Charles's law: V1/T1 = V2/T2; convert temperatures to Kelvin.
  • Volume of 0.72 mol helium gas under same conditions as 0.48 mol occupying 11.7 L
    Use Avogadro's law: volume proportional to moles; calculate volume for 0.72 mol.