Physics quiz question: mass, moles and molar mass of He

Physics 205A Quiz 7, Fall Semester 2008
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 1/e, Problem 13.36

[3.0 points.] How many grams of He atoms are there in 5.0 moles?
(A) 0.80 g.
(B) 1.3 g.
(C) 6.8 g.
(D) 20 g.

Correct answer: (D)

The molar mass of He is 4.0 unified atomic mass units; that is, there are 4.0 g per mole of He atoms. Since there are 5.0 moles of He atoms, then there are 20 g of He atoms. Response (A) is 4.0/5.0; response (B) is 5.0/4.0; response (C) is 4.0*1.7 (in reference to another sample of 1.7 moles of Ne atoms discussed in a later, related question).

Student responses
Sections 70854, 70855
(A) : 6 students
(B) : 5 students
(C) : 2 students
(D) : 23 students

"Difficulty level": 63%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.49