Posted by sara on .
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^{6}meters.
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.

ap chem 
Andrew,
1x10^6 divided by 6.02x10^23 equals 6.02x10^17
1.25x10^20 divided by 6.02x10^23 equals 2.08x10^4