How many molecules are in 0.0913 moles of Selenium?

Answer 1

5.50 x 10^22 particles of selenium. One caution before the arithmetic, because it is the only real chemistry in this problem: elemental selenium does not come in molecules the way water or carbon dioxide does. The stable gray form is built from long helical chains of Se atoms, so it has no molecular formula at all. When a problem says 0.0913 moles of Selenium it means 0.0913 moles of Se ATOMS, and what you are actually counting are atoms.

The conversion itself is one step. Avogadro's number is the count of particles in exactly one mole:

1 mol = 6.022 x 10^23 particles

0.0913 mol Se  x  (6.022 x 10^23 atoms / 1 mol)  =  5.4981 x 10^22 atoms

Rounded to the three significant figures carried by 0.0913, that is 5.50 x 10^22 atoms.

Here is a second route that puts the arithmetic in a different place, so agreement is a real check rather than the same chain run twice. Ask what fraction of a mole you have, then take that fraction of Avogadro's number:

0.0913 mol is 9.13 percent of a mole

6.022 x 10^23  x  0.0913  =  5.4981 x 10^22

Three wrong answers this problem reliably produces, each one diagnosable:

1.52 x 10^-25. This is dividing by Avogadro's number instead of multiplying, and you can kill it before doing any arithmetic at all. A mole is already a count of 6 x 10^23 things, and you have about a tenth of a mole, so the answer has to be an enormous number somewhat smaller than 6 x 10^23. Anything that comes out with a negative exponent went the wrong direction.

7.21 g. This comes from reaching for the molar mass, 78.97 g/mol. It is a correct calculation of the wrong thing: the problem already handed you moles, so the molar mass has no job here. Molar mass is the bridge between grams and moles. Avogadro's number is the bridge between moles and particles. Ask which bridge the question actually put you on.

6.87 x 10^21. This is the interesting error, because it comes from doing something that feels careful. Sulfur famously exists as S8 rings, and selenium does have an Se8 allotrope of its own (the red form), so a student who remembers that divides by 8 to turn atoms into rings. What makes it wrong is a convention rather than a fact about selenium: for an element, moles of X means moles of X atoms unless the problem writes the molecular formula out explicitly. Had it said 0.0913 mol Se8, dividing by 8 would have been exactly right.

Two closing notes. Avogadro's number never limits your significant figures, because since the 2019 SI redefinition 6.02214076 x 10^23 is an exact defined value; all three of your sig figs come from 0.0913. And for scale, 5.50 x 10^22 atoms of selenium is 7.21 g, a small spoonful of dark gray powder, and roughly 360 times what an adult needs to eat in an entire year at 55 micrograms a day. Selenium is an essential trace element with an unusually narrow window between too little and toxic, which is why nobody measures it out in grams.