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MCAT · Chemical and Physical Foundations · General Chemistry
Gases is part of General Chemistry in the MCAT Chemical and Physical Foundations section. The questions below are written in the style of the real test, each with the answer and a worked explanation.
Question types you will see here include ideal gas under changed conditions, dalton's law and partial pressures, gas stoichiometry and graham's law of effusion.
Common traps: using Celsius instead of kelvin; inverting a proportionality; assuming a rigid container expands.
Question 1
Using R ≈ 0.08 L·atm/(mol·K), approximately how many moles of an ideal gas occupy 16 L at 3 atm and 300 K?
0.16 mol
0.5 mol
2 mol
180 mol
Answer: C. 2 mol
n = PV/RT = (3)(16)/(0.08·300) = 48/24 ≈ 2 mol. Inverting the ratio gives 0.5.
Question 2
Under identical conditions, an unknown gas effuses through a porous plug at one-third the rate of helium (molar mass 4.0 g/mol). What is the molar mass of the unknown gas?
0.44 g/mol
1.3 g/mol
12 g/mol
36 g/mol
Answer: D. 36 g/mol
rate(He)/rate(X) = √(M_X/M_He), so 3 = √(M_X/4.0) and M_X = 4.0 × 9 = 36 g/mol (D). C forgets to square the rate ratio, A inverts the relation as if slower gases were lighter, and B both inverts and forgets the square.
Question 3
At moderately high pressure and room temperature, the measured pressure of ammonia in a cylinder is lower than the ideal gas law predicts. What best explains this?
Attractions between molecules reduce the force of their collisions with the walls.
The molecules' own volume reduces the free space available to them.
Ammonia molecules are heavier than the point particles assumed by the ideal model.
Collisions between molecules are inelastic and steadily drain kinetic energy.
Answer: A. Attractions between molecules reduce the force of their collisions with the walls.
Polar NH₃ molecules attract each other (the van der Waals a term), so molecules about to strike a wall are pulled back and the pressure falls below ideal (A). B describes the b term, which raises pressure above ideal; C confuses mass with nonideal behavior; D would make the gas cool continuously, which does not happen at constant temperature.
Question 4
A wound-cleaning solution contains 3.4 g of H₂O₂ (34 g/mol), which decomposes completely by 2H₂O₂ → 2H₂O + O₂. What volume of O₂ forms at 1.0 atm and 300 K, where one mole of gas occupies 24.6 L?
1.2 L
2.5 L
42 L
84 L
Answer: A. 1.2 L
3.4 g/34 g/mol = 0.10 mol H₂O₂, which gives 0.050 mol O₂ (2:1), occupying 0.050 × 24.6 ≈ 1.2 L (A). B ignores the 2:1 mole ratio, C treats grams as moles before applying the ratio, and D treats grams as moles and also ignores the ratio.
Question 5
A heliox cylinder for a patient with airway obstruction contains 16 g of O₂ (32 g/mol) and 6.0 g of He (4.0 g/mol) at a total pressure of 2.0 atm. What is the partial pressure of oxygen?
0.50 atm
1.0 atm
1.5 atm
2.0 atm
Answer: A. 0.50 atm
Moles are 16/32 = 0.50 mol O₂ and 6.0/4.0 = 1.5 mol He, so the O₂ mole fraction is 0.25 and P(O₂) = 0.25 × 2.0 atm = 0.50 atm (A). C uses the mass fraction 16/22, B splits the pressure equally between two gases, and D assumes each gas exerts the full total pressure.
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