Which sample of fluorine gas, F2, contains the greatest number of fluorine molecules? (multiple choice)

A. 1.9 x 1023 F2 molecules.
B. 1.9 dm3 of F2 at 273 K and 1.00 x 105 Pa pressure.
C. 1.9 g F2
D. 1.9 mol F2

The answer is D. 1.9 mol F2, why? How do you calculate it?

1 Answer
Feb 16, 2016

Let's use the symbol NA, where NA = 6.022×1023.

Explanation:

A. there are approx. 13NA, INDIVIDUAL F2 molecules.

B. there are approx 112NA, INDIVIDUAL F2 molecules.

C. there are approx. 119NA, INDIVIDUAL F2 molecules.

D. there are approx. 1.9×NA, INDIVIDUAL F2 molecules.

By using the symbol NA (where NA is Avogadro's number) it is clear that option D is BY FAR THE MOST NUMEROUS.

Option A. was simply a number. To assess B, I know that NA F2 molecules has a volume of 22.4L. To assess C, I know that NA F2 molecules has a mass of 38 g, so in 1.9 g F2 there are approx. 119×NA such molecules. But D. specifies approx. 2NA F2 miolecules.

So, when we use Avogadro's number as a NUMBER, and indeed it is a number, option D CLEARLY contains the most molecules.

I could have replaced the "mole" term with a dozen (because that's the number the mole is equivalent to!) and got nearly the same answer. If you are unconvinced with this treatment, state your objections, and we will try again. The point is that NA is simply a number, that has the property that NA F2 molecules has a mass of 38.0 g.