Answer:
7.5 gm left
Explanation:
Bismuth-210 has a half life of 5 days
15 days is 15/5 = 3 half lives
since half the amount is left in 5 days or 1 half life
(1/2) x (1/2) x (1/2) the staring amount would be left in
3 half lives. so 1/8 is left
(1/8) x 60.0 = 7.5 gm left
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Answer:


Explanation:
1 mol of nitrogen at STP = 22.4 L = 22400 cc
n = Mol of
= 
M = Molar mass of
= 
= Avogadro's number = 
Mass of
is

Mass of the nitrogen is 
Number of molecules is given by

The number of molecules present in it are 
You will need a periodic table to help you answer this problem. The atomic numbers are arrange from lowest to highest in the periodic table. You can locate element number 55 to be Cesium with an atomic weight of 132.905 amu. So, you start from element 56. The following elements are:
56 Barium 137.328 amu
57 Lanthanium 138.905 amu
58 Cerium 140.116 amu
59 <span>Praseodymium 140.908 amu
60 Neodymium 144.243 amu
Neodymium is already greater than 144 amu. Therefore, these elements only include Barium, Lanthanium, Cerium and Praseodymium.</span>