N= N_0 e^-kt
t= 36 years
k= ln(2)/12
N= 1 kg
solve for N sub 0.
or think of it this way. you've got one kg of a substance with a half life of 12 years. every 12 years, the amount is cut in half. 36 years is 12 half lives. so half of 1 is .5. half of .5 is .25. and another half is .125.
.125...
density is mass/volume.
.785= x/ 5
this will give you 3.925 grams. divide by the molar mass of t-butyl alcohol which is 74.122 g/mol. that gives you .052953 moles. or .0530 if you're doing correct significant figures.