Chem
posted by Lu .
1.75 x 10^25 selenium atoms contains how many moles of selenium?

1 mol contains 6.02E23 atoms.

I am still not understanding.

Set up a proportion if you don't understand it any other way. For example, you have 5 oranges. They weigh 8 lbs. How much does one weigh?
(8 lbs/5 oranges) = (?lb/1 orange)
Solve for ? = 8*(1/5) = 1.6 lb each.
(1.75E25 atoms/?mol) = (6.02E23 atoms/1 mol).
? mol = 1.75E23 x (1/6.02E23) = ?
Any problem that is directly proportional can be done this way. Another way is to use dimensional analysis.
You know 1 mol contains 6.02E23 atoms. You want to know how many mols in 1.l75E25 atoms.
1.75E25 x conversion factor to convert atoms to mols.
1.75E23 atoms x (1 mol/6.02E23 atoms = ?
Note that atoms in the numerator cancel with atoms in the denominator and leaves mols (w3hich is what you wanted). Dimensional analysis always works.