A new element, X, is discovered. This element forms a compound XF4 in which the percent by mass of fluorine is 16.1 %. What is the atomic mass of X, in amu?

Question

AMU problem 

Answer 1

The atomic mass of fluorine is 18.998 amu, so if you have 4 fluorine, the mass of fluorine in the compound is 4*18.998 or 265.972 amu.

 

If 16.1% of the compound is fluorine, then 16.1% of the total mass is 265.972. To find the total mass of the compound, divide 265.972 by 0.161; this gives you 1652 amu.

 

(Let x represent the total mass; 16.1% as a decimal is 0.161. Therefore 0.161x=265.972. Solve for x, which is 1652).

 

Since 16.1% of the compound is fluorine, the remaining 83.9% is X, and 83.9% of 1652 is 1386.028 amu. You only have one X in this compound, so this is the atomic mass of X.