Mass and Moles [Chemistry]?

Question

I was wondering if someone can help me with this problem now:
A. Calculate the mass of 500. atoms of iron [Fe]. [three sig figs]
B. How many Fe atoms and how many moles of Fe atoms are in 500.0 g of iron? [4 sig figs]

Please keep in mind of the Significant Figures [sig figs] in both parts.
Can someone try and explain how one would approach this problem? =/ Thanks in advance to anyone who can help. :]

Answer 1

A mole of anything has Na (Avogadro's number, 6.02486E23) of atoms and a weight in grams equal to the molecular weight. Since the molecular weight of iron is 55.85 g/mole, the weight of 500 atoms is (1 atom = 1 molecule for iron)

(A) (500 / 6.02486E23) * 55.85 = 4.635E-20 g

(B) (a) 500g * Na = _____atoms (b) 500/55.85 = ___ moles

I'll let YOU worry about significant figures; what's your hangup on that?

Answer 2

I won't give you the answer, but I hope I can help you figure out how to do it.

For A:
First, calculate the number of moles of iron is 500 atoms. (Recall 1 mole = 6.02 x 10^23 atoms).
Then use the molar mass of iron (55.9 g/mol) to calculate how many grams it weighs.
Your final answer should have three significant digits (from "500 atoms", 6.02, and 55.9).

For B:
It's basically the opposite calculation. First divide the 500.0 g of iron by the molar mass of iron (g/mol) to get the number of moles of Fe atoms, then use Avogadro's number to calculate the number of atoms.
Hope this helps.

Answer 3

B. mass/R.A.M. = 500/56=

8.92857142857142 etc.

For the number of atoms it is:
A mole is 6x10 23 (23 is up) or 600, 21zeros

So, 500x600,000,000,000,000,000,000,000 equals 300, 24zeros