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Quizzes > Science

Hard Chemistry Questions: Can You Solve Our Toughest Quiz?

Quick quiz to test your knowledge with difficult chemistry questions. Instant results.

Editorial: Review CompletedCreated By: Virginie JetteUpdated Aug 28, 2025
Difficulty: Moderate
2-5mins
Learning OutcomesCheat Sheet
Paper cut style chemistry quiz banner with molecules lab glassware question marks on dark blue backdrop

This quiz helps you tackle hard chemistry questions and spot gaps before an exam. Work through multi-step problems on rates, acids and bases, redox, and structure, then check your score instantly. If you want a refresher first, try our general chemistry quiz or review basics with basic chemistry questions; when you are done, dig deeper with a chemistry solutions quiz.

What is the atomic number of carbon?
6
8
4
12
The atomic number of an element equals its number of protons. Carbon has six protons in its nucleus, so its atomic number is 6. This distinguishes carbon from other elements on the periodic table. .
Which of the following is a noble gas?
Fluorine (F?)
Argon (Ar)
Oxygen (O?)
Nitrogen (N?)
Noble gases are elements in group 18 of the periodic table, characterized by full valence shells and very low reactivity. Argon is one of these inert gases. It is used in lighting and welding due to its stability. .
What is the molar mass of water (H?O)?
20.04 g/mol
16.00 g/mol
18.02 g/mol
17.01 g/mol
The molar mass of water is calculated by summing the atomic masses: 2×1.008 (H) + 16.00 (O) ? 18.02 g/mol. This value is essential for stoichiometric calculations in reactions involving water. .
Which orbital is shaped like a sphere?
s orbital
p orbital
f orbital
d orbital
The s orbital is the only atomic orbital that is spherically symmetric around the nucleus. p, d, and f orbitals have more complex shapes with angular nodes. Understanding orbital shapes helps explain bonding and molecular geometry. .
What is the oxidation state of oxygen in H?O? (hydrogen peroxide)?
-1
-2
+1
+2
In hydrogen peroxide, each hydrogen is +1, so the two oxygens together must be -2 total, giving each oxygen an oxidation state of -1. This differs from most oxides, where oxygen is -2. .
Calculate the pH of a 0.01 M HCl solution.
3.00
4.00
1.00
2.00
HCl is a strong acid and dissociates completely: [H?] = 0.01 M. pH = -log??[H?] = -log??(0.01) = 2.00. Accurate pH calculations are vital for acid - base chemistry. .
Which of the following compounds exhibits hydrogen bonding?
CH?
CO?
CH?OH
C?H?
Methanol (CH?OH) contains an - OH group where hydrogen is bonded to a highly electronegative oxygen, enabling hydrogen bonding. This intermolecular force significantly raises its boiling point compared to similar-sized molecules. .
What is the limiting reactant when 3 moles of H? react with 1 mole of O? to form water?
H?
O?
None, they are stoichiometric
H?O
The balanced reaction is 2 H? + O? ? 2 H?O. For 1 mole of O? you need 2 moles of H?. Here you have 3 moles of H?, so H? is in excess and O? is limiting. Identifying the limiting reagent is key in yield calculations. .
What is the formal charge on the central nitrogen in NH??
+2
-1
0
+1
Formal charge = valence electrons - (nonbonding electrons + ½ bonding electrons). Nitrogen has 5 valence electrons, 2 are in a lone pair, and it shares 6 electrons in bonds, so FC = 5 - (2 + 3) = 0. Formal charges help predict stability. .
The reaction N? + 3H? ? 2NH? has ?H° = - 92 kJ. Increasing the temperature will shift the equilibrium toward:
No change in position
It depends on pressure
The reactants N? and H?
The production of NH?
For an exothermic reaction (?H < 0), adding heat shifts equilibrium toward the reactants (Le Chatelier's principle). Thus raising temperature favors N? and H?. This principle is vital in industrial ammonia synthesis. .
Given ?G° = -40 kJ at 298 K for a reaction, what is the equilibrium constant K?
1.0 × 10??
1.0 × 10³
2.3 × 10²
1.0 × 10?
Use ?G° = - RT ln K. Rearranging gives K = exp( - ?G°/RT). Substituting ?G° = - 40000 J, R = 8.314 J·mol?¹·K?¹, T = 298 K yields K ? 1×10?. Equilibrium constants indicate reaction favorability. .
What is the wavelength of the photon emitted when an electron in a hydrogen atom transitions from n=3 to n=1?
121.6 nm
97.3 nm
102.6 nm
656.3 nm
Using the Rydberg formula 1/? = R?(1/1² ? 1/3²), where R? = 1.097×10? m?¹, yields ? ? 1.026×10?? m or 102.6 nm. This is the Lyman-? line in the ultraviolet. .
Determine the pH of a buffer made from 0.10 M acetic acid (pKa = 4.76) and 0.10 M sodium acetate.
7.00
4.76
4.00
5.00
For a buffer, Henderson - Hasselbalch: pH = pKa + log([A?]/[HA]). Here [A?] = [HA], so pH = pKa = 4.76. Buffer pH control is crucial in biochemistry and industrial processes. .
Identify the hybridization of the carbon atoms in ethene (C?H?).
sp
sp²
sp³
dsp²
Each carbon in ethene forms three sigma bonds and one ? bond, requiring sp² hybridization (three sp² orbitals and one unhybridized p orbital). This geometry results in a planar structure with 120° bond angles. .
In the redox reaction MnO?? + C?O?²? ? Mn²? + CO? (acidic medium), how many electrons are transferred per balanced equation?
6
10
2
5
Balancing half-reactions in acidic solution shows permanganate gains five electrons per Mn, and oxalate loses two electrons per C?O?²?. Scaling gives 10 total electrons exchanged. Electron counting is key in titrations and redox analysis. .
What volume of 0.50 M NaOH is required to completely neutralize 25.0 mL of 0.20 M H?SO??
10.0 mL
5.0 mL
20.0 mL
40.0 mL
Moles H?SO? = 0.025 L × 0.20 M = 0.005 mol. Neutralization requires 2 mol NaOH per mol H?SO?, so 0.010 mol NaOH. Volume = 0.010 mol ÷ 0.50 M = 0.020 L (20.0 mL). Accurate titration calculations are fundamental. .
Calculate the rate constant k at 298 K for a reaction with activation energy Ea = 75 kJ/mol and frequency factor A = 1.0×10¹² s?¹ using the Arrhenius equation.
3.0×10?? s?¹
2.5×10?¹ s?¹
1.0×10³ s?¹
7.3×10?² s?¹
Arrhenius: k = A·exp( - Ea/RT). Substituting A = 1×10¹² s?¹, Ea = 75,000 J/mol, R = 8.314 J·mol?¹·K?¹, T = 298 K gives k ? 7.3×10?² s?¹. This factor strongly depends on temperature. .
How many stereoisomers exist for 2,3-dichlorobutane?
2
4
5
3
2,3-Dichlorobutane has two chiral centers but also a meso form due to internal symmetry, yielding three stereoisomers (one meso and one pair of enantiomers). Counting stereoisomers is crucial in stereochemistry. .
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Study Outcomes

  1. Understand Core Chemical Principles -

    Identify and recall foundational concepts such as chemical bonding, molecular structure, and periodic trends to tackle the hardest chemistry question quiz.

  2. Apply Stoichiometry to Difficult Chemistry Problems -

    Use mole-to-mole calculations, limiting reagents, and percent yield analyses to solve complex reaction equations accurately.

  3. Solve Advanced Thermodynamics and Kinetics Calculations -

    Compute enthalpy changes, equilibrium constants, and reaction rates in challenging scenarios to deepen your grasp of chemical energetics.

  4. Analyze Reaction Mechanisms and Equilibrium Scenarios -

    Break down multi-step mechanisms, predict reaction intermediates, and evaluate Le Chatelier's principle in dynamic systems.

  5. Evaluate Real-World Chemistry Challenges -

    Apply theoretical knowledge to practical problems and case studies, bridging the gap between classroom concepts and real-world applications.

  6. Assess Mastery with a Scored Chemistry Practice Test -

    Gauge your proficiency through a timed trivia chemistry quiz featuring the toughest chemistry questions for self-evaluation and targeted review.

Cheat Sheet

  1. Gibbs Free Energy and Spontaneity -

    Mastering ΔG = ΔH - TΔS is essential for tackling the hardest chemistry question on reaction spontaneity (NIST). Practice calculating Gibbs free energy changes at different temperatures and use the mnemonic "Gotta Have Some Temperature" to recall G = H - TS. Understanding how ΔG sign predicts equilibrium will help you ace difficult chemistry problems in real-world scenarios.

  2. Chemical Kinetics and Rate Laws -

    Derive rate laws from experimental data and apply the method of initial rates to determine reaction order, a key skill in any chemistry practice test (ACS Resources). Remember the mnemonic "Zero, First, Second - ZFS" to keep rate law forms straight. Work through sample problems calculating half-lives for first-order and second-order reactions to build confidence.

  3. Equilibrium Concepts and Le Châtelier's Principle -

    Understand Kc and Kp expressions and how changes in concentration, pressure, or temperature shift equilibria (MIT OpenCourseWare). Flashcards are great for memorizing how adding reactants or increasing temperature affects endothermic vs. exothermic systems. Being fluent in equilibrium calculations is crucial for tough chemistry questions on shift predictions.

  4. Acid-Base Titration Curves -

    Analyze pH changes during titration of strong/weak acids and bases using the Henderson-Hasselbalch equation (Khan Academy). Plotting titration curves and identifying equivalence points will prepare you for trivia chemistry quiz items on buffer regions. Try practice problems adjusting Ka or Kb values to see how curve shape varies.

  5. Organic Reaction Mechanisms -

    Distinguish SN1 vs. SN2 pathways by examining nucleophile strength, substrate structure, and solvent effects (IUPAC Guidelines). Use the "Backside Attack" mental image for SN2 and the "Carbocation Stability Ladder" for SN1 to recall key features quickly. Applying these patterns solves many of the most difficult chemistry problems in organic synthesis.

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