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

1 Answer
Sep 13, 2017

Answer:

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"#?