2025 USNCO Chemistry Olympiad: Practice Questions and Solutions

What is the gas constant R in J mol-1 K-1?

8.314 J mol-1 K-1

What is the Faraday constant F in C mol-1?

96,500 C mol-1

What is Avogadro's number N_A?

6.022 × 10^23 mol-1

What is Planck's constant h in J s?

6.626 × 10-34 J s

What is the speed of light c in m s-1?

2.998 × 10^8 m s-1

What is the temperature conversion from 0 °C to K?

273.15 K

What is the molar mass of sodium azide (NaN3)?

65.02 g/mol

What is the specific heat capacity of water?

4.184 J g-1 K-1

What is the pressure equivalent of 1 atm in mm Hg?

760 mm Hg

What is the molar mass of lithium hydrogen carbonate (LiHCO3)?

(Li: 6.94 + H: 1.01 + C: 12.01 + 3*O: 16.00) = 61.94 g/mol

What is the formula for the decomposition of sodium azide?

2 NaN3(s) → 2 Na(s) + 3 N2(g)

What is the partial pressure of CO2 in a vessel containing 66 g CO2 and 16 g O2 at 10.0 atm?

5.0 atm

How many moles of H2O are formed when 0.50 mol LiHCO3 decomposes?

0.25 mol

Which hydrocarbon is 84.1% carbon by mass?

C8H18

Which solid is least soluble in water at 298 K?

CaF2

What methods are suitable for determining the concentration of an aqueous solution of KMnO4?

I. Visible spectrophotometry (colorimetry), II. Redox titration

What mass of sodium azide is required to inflate a 16.0 L airbag to a pressure of 1.20 atm at 17 ºC?

34.9 g

What is the concentration of sulfate ion in a solution measured by?

(not specified in the notes)

What is the result of mixing 500 mL of 0.4 M NaOH with 500 mL of 0.4 M HCl?

It will neutralize to form water and salt.

What error results in a higher sulfate concentration than actual?

The empty crucible contains a few drops of water when tared.

Which solute gives the highest boiling point when dissolved in 1 kg H2O?

50 g glyoxal, HCOCHO.

What is the best way to dispense liquids using a volumetric pipet?

The pipet is immersed in the liquid, lifted with a gloved finger, and allowed to drain to the mark.

What type of solid is represented by a lattice containing two different elements X and Y?

A metallic alloy such as FeCr.

Which gas at 600 K has the same effusion rate as methane (CH4) at 150 K?

He.

Why does ammonia (NH3) have a higher boiling point than PH3 or AsH3?

NH3 has extensive hydrogen bonding while PH3 and AsH3 do not.

In which order are the ionic solids ranked in increasing melting point?

KBr < NaCl < NaF < MgO.

What is the final temperature of a mixture of 10.0 g Fe and 10.0 g Ti after losing 200 J of heat?

89.9 ºC.

Which sample has the greatest entropy at a given temperature?

2 mol HBr(g).

What must be true for an endothermic reaction to be spontaneous under standard conditions?

The entropy change ∆Sº must be positive and greater than ∆Hº/T.

What is the difference between internal energy change (∆U) and enthalpy change (∆H) for a constant-pressure process?

Heat.

What is the standard enthalpy of formation of SiF4(g) given the reaction SiO2(s) + 4 HF(aq) → SiF4(g) + 2 H2O(l)?

-1624.3 kJ mol-1.

How may an enzyme inhibitor decrease the rate of a reaction?

The inhibitor may bind to the enzyme and block the active site.

What is ∆Gº at 298 K for the reaction Ba2+(aq) + SO42-(aq) → BaSO4(s)?

-57 kJ mol-1.

What is the rate law for the reaction A + B → products when [B] >> [A]?

Rate = k[A][B].

What is the mechanism consistent with the rate law Rate = k[(CH3)3COH][H+]?

Fast.

For which reaction would the yield of products at equilibrium NOT increase at a higher pressure?

N2(g) + O2(g).

What is the Ksp of BaSO4 at 298 K?

1.1 × 10-10.

What is the Ksp of Chromium(III) hydroxide?

1.6 × 10-30

What is the molar solubility of Cr(OH)3 in a solution with pH 6.00?

1.6 × 10-8 mol L-1

What is Kp at 400 K for the reaction 2 SO3(g) → 2 SO2(g) + O2(g)?

8.2 × 10-4

What is the pH of a 0.10 M solution of NaCN?

8.85

What is the ratio of the coefficient of Ag(s) to the coefficient of H2O(l) in the reaction 2 Ag(s) + Cu2+(aq) + 2 Cl-(aq) → 2 AgCl(s) + Cu(s)?

1 : 1

What is the standard cell potential of the galvanic cell Zn(s) | Zn2+(aq) || Ag+(aq) | Ag(s) at 298 K?

1.602 V

What is the Ksp of Ca(OH)2 at 298 K?

8.4 × 10-6

Which gas-phase atoms in their ground states are diamagnetic?

Zn

Which statements correctly describe the geometry of the carbonate ion, CO32-?

I. All three carbon-oxygen bond distances are the same. II. All three bond angles are 120º.

What reaction conditions most effectively convert a carboxylic acid to a methyl ester?

CH3OH, HCl

Which molecule has a trigonal pyramidal geometry?

PCl3

How many σ bonds and how many π bonds are present in allene, H2CCCH2?

Five σ, one π

What mode of radioactive decay does 66Cu undergo?

Beta decay

Which compound can exhibit geometric isomerism?

2-butene

What is the maximum number of monosubstitution products of the aromatic substitution reaction shown?

2

What is the oxidation state of ruthenium in the nitrate salt?

+4

What is the bond dissociation enthalpy comparison between F2 and Cl2?

The bond in F2 has a smaller bond dissociation enthalpy than the bond in Cl2 due to stronger lone pair-lone pair repulsions.

Which aspect of metallic bonding best accounts for the characteristic malleability of metals?

Metals contain delocalized electrons.

Which structural features contribute to the low strain energy of cyclohexane?

All bond angles are close to 109.5º and all C-H bonds on adjacent carbon atoms are staggered.

What is ∆Eº for the reaction involving Ag and Cu?

-0.107 V

What is the pKa of RCOOH?

4.2

What is the boiling point trend of alkali metals?

Decreases from lithium to cesium.

What is the gas constant R in L bar mol-1 K-1?

0.08314 L bar mol-1 K-1

What is Faraday's constant F in C mol-1?

96,500 C mol-1

What is Faraday's constant F in J V-1 mol-1?

96,500 J V-1 mol-1

What is Planck's constant h?

6.626 × 10-34 J s

What is the speed of light c?

2.998 × 10⁸ m s-1

What is the temperature conversion for 0 °C in Kelvin?

273.15 K

What is the pressure equivalent of 1 atm?

1.013 bar = 760 mm Hg

What is the specific heat capacity of H2O?

4.184 J g-1 K-1

What is the abbreviation for the amount of substance?

n

What is the abbreviation for ampere?

A

What is the abbreviation for atmosphere?

atm

What is the abbreviation for atomic mass unit?

u

What is the abbreviation for density?

d

What is the abbreviation for enthalpy?

H

What is the abbreviation for entropy?

S

What is the abbreviation for molar mass?

M

What is the abbreviation for pressure?

P

What is the abbreviation for temperature?

T

What is the abbreviation for time?

t

What is the abbreviation for volume?

V

What is the abbreviation for frequency?

ν

What is the abbreviation for joule?

J

What is the abbreviation for kelvin?

K

What is the abbreviation for liter?

L

What is the abbreviation for mole?

mol

What is the abbreviation for gram?

g

What is the abbreviation for second?

s

What is the abbreviation for volt?

V

What is the abbreviation for vapor pressure?

VP

What is the abbreviation for free energy?

G

What is the empirical formula of a compound that is 50% S and 50% O by mass?

SO

What should be used for the final rinse of a buret filled with 0.100 M NaOH?

Distilled water

How much chlorine is required to completely consume 13.5 g of aluminum to form aluminum chloride?

26.6 g

In which solvent does solid iodine dissolve to give a purple solution?

Carbon tetrachloride

What is the concentration of iodide in a solution mixed from 0.500 L of 0.150 M KI and 0.250 L of 0.200 M CaI2?

0.175 M

What percentage by mass of the ore is malachite if 47.0 g of metallic copper is obtained from a 1.00 kg sample?

4.70%

Which aqueous solution has the highest boiling point when 10.00 g of the substance is dissolved in 100.0 g of water?

H2SO4

Which compound could be used to remove Ca2+ from hard water?

Na2CO3

What is the molecular formula of a gas that occupies a volume of 264 mL at 722 mm Hg and 22 °C?

C2F6

What is the Ksp of AgBr at 298 K?

3.1 × 10^-14

How many nearest neighbors does a cadmium atom have in a hexagonal closest packed structure?

12