What molar quantity is represented by a 5.75*g mass of sodium?

1 Answer
anor277
Sep 13, 2017

For all these problems we take the quotient.....

Explanation:

"Number of moles" = "Mass of stuff"/"Molar mass of stuff"

Where 1*mol-=6.022xx10^23*"individual items of stuff".

"Moles of sodium" = (5.75*g)/(22.99*g*mol^-1)=0.250*mol.

And note the dimensional consistency of the answer....i.e.

(5.75*cancelg)/(22.99*cancelg*mol^-1)=0.250*1/(1/(mol))=0.250*mol.

How did I know the molar mass of sodium? How will you know it if you are asked in an exam?

And to get the molar mass of sodium, chlorine, and copper, I look at that thing that tells me the molar masses of these elements. I forget its name....and I get molar masses of 22.99*g*mol^-1, Na; 35.45*g*mol^-1, Cl; 63.55*g*mol^-1, Cu.

Note that chlorine is commonly encountered as the diatomic molecule, i.e. Cl_2, but given the terms of the question, I feel I am justified in specifying a mole of chlorine ATOMS......But what would be the mass of 6.022xx10^23*"chlorine molecules"?

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