Question

How would you calculate the number moles of calcium in 27.0 g of Ca?

`27.0 g Ca x (1 mol Ca)/(6.022 x 10^23 Ca) = ?`

Answer 1

Simply divide thru by the molar mass.........and multiply by `N_A`

Explanation:

We KNOW that `"Avogadro's Number"` of calcium atoms has a mass of `40.1*g`. How do we know this? Well, it's printed on the Periodic Table that should be beside you now if you are doing your chemistry homework.

And thus we can use this mass as a measure of the NUMBER of calcium atoms we possess.......i.e.

`"Number of calcium atoms"=(27.0*g)/(40.1*g*mol^-1)xxN_A`, where `N_A="Avocado's number"=6.022xx10^23*mol^-1`.

`=(27.0*cancelg)/(40.1*cancelg*cancel(mol^-1))xx6.022xx10^23*cancel(mol^-1)`.

And we get the answer as a DIMENSIONLESS NUMBER as required.