Chemistry

Which units could be used to express the amount of energy absorbed or released during a chemical reaction?

(A) Degree and gram
(B) Torr and mmHg
(C) Gram and liter
(D) Calorie and joule
(E) Meter and cm3
Chemistry

Which pair of symbols below show different isotopes of the same element?

(A) 3918A and 3919R
(B) 6027X and 5928Y
(C) 126L and 146L
(D) 3717X and 3717X
(E) 32E and 31G
Chemistry

Which sentence below is incorrect?

(A) Salads are heterogeneous mixtures.
(B)  NaCl(aq) is a homogeneous mixture.
(C) Milk is a heterogeneous mixture.
(D) Sand and water make a heterogeneous mixture.
(E) Pure iron is a heterogeneous mixture.
Chemistry

2019 U.S. National Chemistry Olympiad Local Section Exam Question 20

Which sample has the greatest entropy at a given temperature?

(A) 2 mol HBr(s)
(B) 2 mol HBr(l)
(C) 2 mol HBr(g)
(D)  1 mol H2(g) + 1 mol Br2(l)
Chemistry

2019 U.S. National Chemistry Olympiad Local Section Exam Question 9

Which solid is least soluble in water at 298 K?

(A) CaF2
(B) AgF
(C) Ba(OH)2
(D) CoSO4
Chemistry

A liquid will boil when

(A) its freezing point is equal to its melting point
(B) a salt has been added to the liquid
(C) its vapor pressure is equal to the melting point
(D) it is heated to a temperature that is below the boiling point
(E) its vapor pressure is equal to the surrounding pressure
Chemistry

Which gas is expected to have the highestrate of effusion?

(A) O2
(B) F2
(C) H2O
(D) He
(E) CH2
Chemistry

2019 U.S. National Chemistry Olympiad Local Section Exam Question 12

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

(A) The pipet is immersed in the liquid to be dispensed, then lifted with a gloved finger on the top and the liquid allowed to drain to the mark. The remaining contents are then allowed to drain into the desired container.
(B) The pipet is immersed in the liquid to be dispensed, then lifted with a gloved finger on the top and the liquid allowed to drain to the mark. The remaining contents are then allowed to drain into the desired container, with a pipet bulb used to gently blow out any residual droplets.
(C) The tip of the pipet is submerged below the surface of the liquid and suction is applied using a pipet bulb until the liquid rises above the level of the mark. A gloved finger is then applied to the top of the pipet and the liquid allowed to drain to the mark. The remaining contents are then allowed to drain into the desired container.
(D)  The tip of the pipet is submerged below the surface of the liquid and suction is applied by mouth until the liquid rises to the level of the mark. The contents are then allowed to drain into the desired container.
Chemistry

2019 U.S. National Chemistry Olympiad Local Section Exam Question 16

A two-dimensional slice through the lattice of a crystalline solid containing two different elements X and Y is shown schematically and to scale below. What type of solid is it?

(A) A metallic alloy such as FeCr
(B) A molecular solid such as IBr
(C) An ionic compound such as LiCl
(D) A network covalent solid such as SiC
Chemistry

2019 U.S. National Chemistry Olympiad Local Section Exam Question 17

A portion of the phase diagram of uranium hexafluoride (UF6) is shown below.

Which statements are correct?

I.  UF6 sublimes at atmospheric pressure.
II. At 80 ºC and 1.5 atm, only UF6(g) is present at equilibrium.
(A) I only
(B) II only
(C) Both I and II
(D) Neither I nor II