Whatis the simplest formula of a rhenium oxide that is 85.33 % Re by mass? Write the symbol of the metal firs?

Question

Where do I Begin?

Answer 1

If the formula contains only 2 elements (Re and O) and it is 85.33% Re, then it must contain 14.67% O

Let's assume that you have 100 grams of this compound:

85.33 g Re and 14.67 g O

Now let's convert the grams into moles by dividing by the atomic masses for each element:

Re = 186.21

O = 16.00

(85.33/186.21) = 0.458 mol Re

(14.67/16.00) = 0.917 molO

Hopefully you can see that the mole ratio of O to Re is 2-to-1. Therefore the simplest formula is ReO2.

Of course, you could always "guess-and-check" your way to ReO2, which has a molar mass of 218.21 g/mol. When you calculate the mass percent of Re (186.21/218.21) you would get 85.33%

Answer 2

Begin by assuming that you have 100 grams of the metal oxide.  In that case there will be 85.33 grams of Re and 14.67 g of oxygen.

Then covert the masses to moles.

85.33 g Re x (1 mol Re / 186.2 g Re) = 0.4583 mol Re

14.67 g O x (1 mol O / 16.0 g O) = 0.9169 mol O

Divide both by the smaller number of moles and get

1 mol Re and 2 mol O

The formula is ReO2