# Chemistry and Exponential Models

• March 25th 2009, 01:58 PM
cheer4life1694
Chemistry and Exponential Models
Radium-221 has a half-life of 30 seconds. How long will it take for 77% of a sample to decay?
• March 25th 2009, 02:31 PM
skeeter
Quote:

Originally Posted by cheer4life1694
Radium-221 has a half-life of 30 seconds. How long will it take for 77% of a sample to decay?

y = fraction of substance left after time t

h = half-life in same units as t

$y = \left(\frac{1}{2}\right)^{\frac{t}{h}}$