Thallium-208 has a half life of 3.053 minutes. How much of a 78.7g sample is left after 25 minutes?

0.31 g

Explanation:

N/No = (1/2)^t/t1/2

N= Amount of radioactive isotope at time t

No= Amount of radioactive isotope originally present

t= time elapsed = 25 minutes

t1/2 = 3.053 minutes

N/78.7 = (1/2)^25/3.053

N/78.7 = (1/2)^8

N/78.7 = 1/256

256 N = 78.7

N= 78.7/256

N= 0.31 g

it should be p1 v1 = p2 v2

c. heat could easily travel through the molecules in the air and escape/enter the flask. removing the air slows down the heat transfer.