Physics quiz question: moles of H_2

Physics 5A Quiz 7, Spring Semester 2008
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 1/e, Problem 13.14

[3.0 points.] How many moles of H2 molecules are there in 0.40 g of H_2? The atomic mass of a single hydrogen atom is 1.0 u.
(A) 0.20 moles.
(B) 0.40 moles.
(C) 4.0 moles.
(D) 5.0 moles.

Correct answer: (A)

A mole of H_2 molecules will have a mass of 2.0 g. Thus 0.40 g of H_2 molecules must be less than a mole, and thus corresponds to (0.40 g)*((1 mole)/(2.0 g)) = 0.2 moles. Response (B) is using a molecular mass of 1.0 g per mole; response (B) is 1,000*(0.4), while response (D) is the molecular mass divided by moles.

Student responses
Sections 4987, 4988
(A) : 12 students
(B) : 12 students
(C) : 7 students
(D) : 0 students