Physics quiz question: moles of dry ice

Physics 5A Quiz 7, Fall Semester 2007
Cuesta College, San Luis Obispo, CA

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

[Version 1]

[3.0 points.] How many moles of CO2 are there in 60.0 g of CO2? The atomic masses of carbon and of oxygen are 12.0 u and 16.0 u, respectively.
(A) 0.467 moles.
(B) 0.733 moles.
(C) 1.36 moles.
(D) 2.14 moles.

Correct answer: (C)

The mass of one mole of CO2 is 12.0 u + 2*(16.0 u) = 44.0 u. Then the number of moles of CO2 is (60.0 u)/(44.0 u) = 1.36 moles. Choice (A) is (12.0 u + 16.0 u)/(60.0 u). Choice (B) is (44.0 u)/(60.0 u). Choice (D) is (60.0 u)/(12.0 u + 16.0 u).

Student responses
Sections 0906, 0907
(A) : 3 students
(B) : 1 student
(C) : 11 students
(D) : 2 students

[Version 2]

[3.0 points.] How many moles of CO2 are there in 15.0 g of CO2? The atomic masses of carbon and of oxygen are 12.0 u and 16.0 u, respectively.
(A) 0.341 moles.
(B) 0.536 moles.
(C) 1.87 moles.
(D) 2.93 moles.

Correct answer: (A)

The mass of one mole of CO2 is 12.0 u + 2*(16.0 u) = 44.0 u. Then the number of moles of CO2 is (15.0 u)/(44.0 u) = 0.341 moles. Choice (B) is (15.0 u)/(12.0 u + 16.0 u). Choice (C) is (12.0 u + 16.0 u)/(15.0 u). Choice (C) is (44.0 u)/(15.0 u).

Student responses
Sections 0906, 0907
(A) : 7 students
(B) : 3 students
(C) : 4 students
(D) : 3 students