3. A 6.55-gram sample of an unknown element Q reacts with excess fluorine gas to form 10.35 grams of QF₄. The unknown element is most likely which of the following?

A. P
B. S
C. Xe
D. C
E. Te

FAQ: Isn’t F₂ a diatomic molecule with a molar mass of 38 g/mol?

Molecular fluorine exists diatomically when it is present on its own as F₂ (g), with a molar mass of 38 g/mol. In this problem, there are four individual F atoms within the compound QF₄. Not F₂ molecules because there are no F₂ molecules in QF₄, and not F₄ molecules because there is no such thing as an F₄ molecule.

Each F weighs 19 g/mol, with four F’s per molecule of QF₄. To find the moles of F present, use the molar mass of F, 19 g/mol. Then, account for the four F’s in QF₄ using the mole fraction (1 mol Q/4 mol F).