What is the empirical formula for a compound that is comprised of 54.0% sodium, 8.50% boron, and 37.5% oxygen by mass?
A) Na2B2O3.
B) Na3BO3.
C) Na3B2O2.
D) Na4BO4.

Answer:

B) Na3BO3.

Explanation:

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In this case, since percent compositions are used to identify the empirical formula of an unknown compound, we can assume we have 54.0 g of sodium, 8.50 g of boron and 37.5 g of oxygen, and we compute the moles of each one:

[tex]n_{Na}=54.0g*\frac{1mol}{23g} =2.35mol\\\\n_B=8.50g*\frac{1mol}{11g}=0.773mol\\\\n_O=37.5g*\frac{1mol}{16g}=2.34mol[/tex]

Now, we divide by the moles of boron as those are the fewest:

[tex]Na:\frac{2.35}{0.773}=3\\\\B:\frac{0.773}{0.773}=1\\\\O=\frac{2.34}{0.773}=3[/tex]

Thus, the empirical formula is B) Na3BO3.

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