For 1.9 mole of the radioactive isotope strontium-87, half-life = 2.8 hours,?

Question

how many disintegrations have occurred after 20 minutes?

Answer 1

Formula

N=N0 * e^-kt or in half life N =N0*2^(-t/T)

2.8 hours = 2.8*60= 168minutes

N= N02^(-20/168) =0.9208N0

so the number of deintegrations is

N0-N =N0(0.0792)

in 1 mole you have 6.0210^23 atoms
in1.9 moles 6.0210^23 *1.9 =N0
and 0.0792N0= 1.9*6.0210^23*0.0792=9.0610^22 deintegrations