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?
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).