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?

Question

2963 views around the world

You can reuse this answer
Creative Commons License

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.