Question

Phosphorus-32 is a commonly used radioactive nuclide in biochemical research, particularly in studies of nucleic acids. the half-life of phosphorus-32 is 14.3 days. what mass of phosphorus-32 is left from an original sample of 157 mg of na332po4 after 42.0 days? assume that the atomic mass of 32p is 32.0.

Answer 1

given days = 42 days 
 So, number of half life, n = 42/14.3 = 2.93 
 So, after 42 days, the mass of sodium phosphate sample left = original mass
x (1/2)^n, 
 = 157 mg x (1/2)^2.93 = 20.60 g of Na3PO4 after 42 days  
 Formula mass of Sodium phosphate = 3(23) + 32 + 4(16) amu = 165 amu  
 165 g of sodium phosphate contains 32 g of Phosphorus 
 20.60 g ------------------------------? = 20.60 x 32/ 165 =3.99g of P32