chemistry
posted by Gaby .
A radium226 sample initially contains 0.112mol. How much radium226 is left in the sample after 6400 years. The halflife of radium226 is 1600 years.

long way.
k = 0.693/t_{1/2}
ln(No/N) = kt
No = 0.112 mol
N = unknown
k from above.
t = 6400 years.
shorter way.
0.112*(1/2^n)
0.112*(1/2^6400/1600)
0.112*(1/2^4) = N 
0.04 is the answer

The halflife of Radium 226 is 1600 years. How much Ra 226 remains in a 30.0 g sample after 6400 years?