Matter, Measurement & Energy
A student measures the mass of a sample three times and gets highly consistent but inaccurate values. Identify the type of error present and explain what this implies about precision and accuracy.
What is systematic error; the measurements are precise but not accurate?
Two isotopes differ by two neutrons but have nearly identical chemical behavior. Explain at the subatomic level why.
What is that isotopes differ in nuclear mass but retain the same electron configuration and nuclear charge?
If the frequency of electromagnetic radiation triples, describe quantitatively how wavelength and photon energy change.
What is wavelength ÷3 and energy ×3?
Identify which will have the larger lattice energy: MgO or NaCl—and justify using Coulomb’s law.
What is MgO because it has higher charges and smaller ionic radii?
A reaction has a 92% percent yield. Explain precisely what this means on a particle level.
What is that only 92 of every 100 theoretically possible product particles form?
Converting a density reported in g/cm³ into kg/m³ requires changing both mass and length units. State the numerical multiplying factor needed and explain why.
What is multiplying by 1000? (1 g/cm³ = 1000 kg/m³ because mass ×1000 and volume ÷1,000,000)
An element has two isotopes: ⁶²X (70%) and ⁶⁴X (30%). Without calculating an exact value, state whether the average atomic mass is closer to 62 or 64, and justify.
What is closer to 62 because the heavier isotope contributes less to the weighted mean?
Explain how the photoelectric effect contradicts classical wave theory and supports quantized energy.
What is that electrons are ejected only above a threshold frequency, inconsistent with increasing intensity alone?
Predict whether BF₃ or NH₃ has a permanent dipole moment and explain using molecular geometry.
What is NH₃ (trigonal pyramidal) vs BF₃ (trigonal planar symmetric)?
Explain why ionic compounds that contain nitrate almost always dissolve in water.
What is the high solvation energy and stability of NO₃⁻ reducing lattice energy influence?
A metal block sinks in water but floats in glycerin. Using density concepts, explain the relative magnitudes of these three densities.
What is: ρwater < ρmetal < ρglycerin?
A certain ion has 18 electrons and a +2 charge. Identify all possible elements it could be.
What are Ca, Sc, Ti (Ca²⁺, Sc³⁺, Ti⁴⁺… but only Ca²⁺ specifically gives 18 e⁻ as its common ion)?
An electron transition in hydrogen emits a photon of 434 nm. Identify the transition and explain why only certain wavelengths appear.
What is n=5→2; because electron energies are quantized so only discrete transitions occur?
A molecule has a central atom forming three bonds and one lone pair. Predict its approximate bond angle and explain why it is less than the ideal value.
What is ~107°, reduced due to lone-pair repulsion?
A solution contains HF(aq). List all species present in significant amounts and explain why HF behaves differently from HCl.
What is HF, H⁺, F⁻, and HF₂⁻ because HF is a weak acid with partial ionization?
A sample is heated and undergoes a phase change without a temperature increase. At the particulate level, describe what type of energy changes and why temperature remains constant.
What is an increase in electrostatic potential energy as intermolecular attractions are overcome, while kinetic energy stays constant?
Explain why mass spectra of chlorine show a distinct pattern of two peaks in roughly a 3:1 ratio.
What is the natural abundance ratio of ³⁵Cl:³⁷Cl producing parent ions of different masses?
In a multi-electron atom, explain why 4s fills before 3d, but 3d electrons are often removed first when forming cations.
What is the shift in relative orbital energies due to electron–electron repulsion and penetration effects?
Explain why CO has a much higher bond energy than NO, despite both having 14 electrons.
What is a higher bond order in CO due to MO configuration and less antibonding occupation?
In a redox reaction, chromium in Cr₂O₇²⁻ is reduced to Cr³⁺. Determine the number of electrons transferred per chromium atom.
What is 3 electrons (Cr⁶⁺ → Cr³⁺)?
A student uses dimensional analysis to convert units through six steps but ends with the wrong order of magnitude. Identify two likely conceptual mistakes related to unit prefixes or powers of ten.
What are misplacing prefix exponents (e.g., M vs m) and incorrectly inverting unit ratios?
A compound has a molar mass of ~90 g/mol and contains only carbon and oxygen. Using the pattern in its mass spectrum showing fragments at m/z = 44 and 28, infer a likely structural feature.
What is the presence of CO₂ units or carbonyl fragments?
Rank 2p, 3s, 3p, and 3d orbitals by increasing effective nuclear charge experienced by an electron, and justify the trend using penetration.
What is 3d < 3p < 3s < 2p; more penetration → higher Zeff.
A resonance hybrid of a molecule shows two C–O bonds of equal length, even though one Lewis structure suggests a double bond and one suggests a single bond. Explain.
What is delocalization of electrons produces an averaged bond order between 1 and 2?
A precipitation reaction forms 0.652 g of solid from 25.0 mL of 0.100 M BaCl₂ mixed with excess sulfate. Using reasoning, determine whether the reaction was limited by barium or sulfate before doing the math.
What is barium-limited because Ba²⁺ is the provided species with finite amount?