A substance consisting only of Na, B and H is 60.80% Na and 28.60% B. What is the empirical formula of the substance?

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Answer 1 · 2016-10-07T17:41:22

#NaBH_4#

As is standard with these sorts of problems, we assume a #100*g# mass of compound to calculate the empirical formula:

#Na: (60.80*g)/(22.99*g*mol^-1)# #=# #2.65*mol#

#B: (28.60*g)/(10.81*g*mol^-1)# #=# #2.65*mol#

#H: (10.60*g)/(1.00794*g*mol^-1)# #=# #10.52*mol#

And then we divide thru each mass by the ATOMIC mass of each constituent to give a molar ratio.

How did I know that there were #10.6*g# hydrogen? The question did not give me this percentage composition.

And now we divide thru by the smallest molar quantity, i.e. #2.65*mol#, to give an empirical formula of:

#NaBH_4#

This is sodium borohydride, a very common laboratory reagent.