Question

A mole of helium gas contains 6.02x10^23 helium atoms. How many helium atoms are there in a micromole of helium? How many moles of helium does 1.25x10^20 atoms of helium represent?

~I have no idea how to start or what the value of a micromole is? Constructive help would be great... thanx

If there are 6.02x10^23 atoms in a mole of He atoms, then how many mols must there be in 1.25x10^20 atoms. You know 1.25x10^20 atoms MUST be less than 1 mole. Or look at it another way.
How many mols in 6.02x10^23 atoms? 1.0
How many mols in 3.01x10^23 atoms? 0.5
How many mols in 1.5x10^23 atoms?
How many mols in 0.75x10^23 atoms?
Get the idea? How many mols are there in 1.25x10^20 atoms.

A micrometer is 1x10^-6meters.
A microliter is 1x10^-6 liters.
A micromole must be 1x10^-6 mols. So you want to know the number of atoms in 1x10^-6 mol He atoms. Show your work if you get stuck and I can give you other hints. But I predict you won't need them.

Answer 1

1x10^-6 divided by 6.02x10^23 equals 6.02x10^17
1.25x10^20 divided by 6.02x10^23 equals 2.08x10^-4