Posted by Gaby on Tuesday, May 31, 2011 at 9:15pm.
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.

chemistry  DrBob222, Tuesday, May 31, 2011 at 10:29pm
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 
chemistry  Tanzeel Urooj, Monday, April 22, 2013 at 3:30am
0.04 is the answer

chemistry  Brandy, Monday, July 28, 2014 at 4:10pm
The halflife of Radium 226 is 1600 years. How much Ra 226 remains in a 30.0 g sample after 6400 years?