- Students confused about whether to treat a mole as a number or a quantity of matter
- Students unable to visualize, work with such large numbers
- A 2-mm-long line of atoms contains 6x10^23 atoms
(Horton, C. (2004). Student alternative conceptions in Chemistry. Arizona State University. Retrieved from http://www.daisley.net/hellevator/misconceptions/misconceptions.pdf).
To promote conceptual change in learning the mole concept, conceptual change texts were prepared by the instructor and used with the experimental group in five steps. Some exemplary parts of the conceptual change texts are as follows:\
Step 1: Students were asked to define the term mole. Some of the student answers are given below:
--The atomic or molecular mass of a substance expressed in grams is 1 mole.
--Mass/molar mass or n = m/M (the ratio of mass to molar mass) is 1 mole.
--As many pencils as Avogadro's number (NA) is 1 mole.
--16 gram of oxygen atoms are 1 mole.
All of the statements above are correct, but none of them is the correct definition of the mole. The instructor made the correct definition of mole after receiving these answers from students. "A mole is an amount of substance that contains the same number of elementary units as there are [sup.12]C atoms in 12.00000 g [sup.12]C."
Step 2: In this step, the students were asked to define the atomic mass unit (amu). The following are some of the answers received from the students.
--The mass of any single atom is 1 amu or 1 amu = m/M.
--1 amu=1/NA (correct but not the definition of amu).
All of the statements above are correct, but none of them is the correct definition. The instructor gave the correct definition after receiving these answers from students. "1 amu is 1/12th of the mass of one [sup.12]C atom. According to this definition, the mass of one [sup.12]C atom is 12.00000 amu".
Step 3: The relationship between Avogadro's number, atomic mass unit and mass was explained in the following way: It was determined experimentally that there are as many [sup.12]C atoms as Avogadro's number in 12.00000 gram [sup.12]C, and the mass of one [sup.12]C atom was accepted as 12 amu according to the definition of amu. Given that 12 gram [sup.12]C is 1 mole (according to the definition of mole), the number of [sup.12]C atoms in 12 gram [sup.12]C is 6.02x[10.sup.23] (experimentally calculated). Then,
How many gram is 1 amu? (1 amu = ? gram)
12(gram)=(12x6.02x[10.sup.23]) amu (when both sides are divided by 12), l(gram)=6.02x[10.sup.23] amu, and hence, 1 amu=(1/6.02x[10.sup.23]) gram = 1.66x[10.sup.-24] gram.
Step 4: Relationship between molar mass and relative atomic mass was [sup.12]C Why is the number standing for molar mass in grams for any atom the same with the number standing for the relative mass in atomic mass units for a single atom?
Let us consider an argon atom. The true mass of an argon atom is 10/3 times the true mass of [sup.12]C atom. Namely, the relative mass of an argon atom is 40 amu (12 amu x 10/3=40 amu). According to the definition of mole, 6.02x[10.sup.23] Ar atoms is 1 mol. Hence, the mass of 1 tool Ar atom is equal to 40 x NA amu. Because 1 gram=NA amu, 1 mol of Ar atom is 40 gram. As a result, relative mass of 1 Ar atom is 40 amu and mass of 1 mol Ar atom is 40 gram.
(Uce, M. (2009). Teaching the mole concept using a conceptual change method at college level. BNET Australian edition. Retrieved from http://findarticles.com/p/articles/mi_qa3673/is_4_129/ai_n31948148/?tag=content;col1)