AP Chem
posted by Anna .
Help with these two please.
105. A radioactive metal atom decays (goes to another kind of atom) by emitting an alpha particle (He+2 ion). The alpha particles are collected as helium gas. A sample of helium with a volume of 12.05 mL was obtained at 765 mmHg and 23 Degrees C. How many atoms decayed during the period of the experiment?
108. A hydrocarbon gas has a density of 1.22 g/L at 20 degrees C and 1.00 atm. An analysis gives 80.0% C and 20.0% H. What is the molecular formula?

105. A radioactive metal atom decays (goes to another kind of atom) by emitting an alpha particle (He+2 ion). The alpha particles are collected as helium gas. A sample of helium with a volume of 12.05 mL was obtained at 765 mmHg and 23 Degrees C. How many atoms decayed during the period of the experiment?
Can't you use PV = nRT, solve for n mols He, change that to molecules He. Won't that be the number of atoms that decayed; that is, 1 atom He for each atom of the radioactive material that decayed?
108. A hydrocarbon gas has a density of 1.22 g/L at 20 degrees C and 1.00 atm. An analysis gives 80.0% C and 20.0% H. What is the molecular formula?
Take 100 g sample which gives you
80 g C
20 g H.
Convert to mols C and mols H.
Take the ratio and find the empirical formula. I assume you know how to do that. Then use PV = nRT but plug density instead of n and V. If you substitute g/molar mass for n and grams/v for density, you get P*molar mass = density x R*T.
That will give you the approximate molar mass, divide by the empirical molar mass to get a number,round to the nearest whole number, and use that as a multiplier x the empirical formula to arrive at the molecular formuls.