Question

If I have a sample from doing a half lfe lab with m&ms, not timed, just pulling out decayed or m&ms with rhe m on top, how do I calculate the half life using t1/2=(.693)/k formula. Do I have to do this 9 times as it took 9 trys to use up all the m&ms. I am not really sure what numbers to feed in. Could you please help?
Thank you. We made a graph and a chart with the numbers from the "decayed m&ms".

Answer 1

You need to make a plot of 'undecayed' M&M's vs Throw number. Undecayed M&M's = (UD) =>
UD(1) = (Total M&M's Before 1st throw) - (M's-up after 1st throw)
UD(2) = [UD(1) - (M's up after 2nd throw)]
UD(3) = [UD(2) - (M's up after 3rd throw)]
UD(4) = [UD(3) - (M's up after 4th throw)]
So on... through UD(9)...

=> Then plot UD(#) vs Throw(#) If you know how to use Excel, this works great... If not, just trace best line through points. Then ...

=> On the UD(# axis) find point of (Total M&M's)/2.
=> Draw line parallel to Throw(#) axis until you intersect the trend curve.
=> Draw line down to Throw(#) axis => this is the Half-Life Value.

=> Substitute the Half-Life Value into [0.693/t(1/2)] => Decay constant in units of (Throws)⁻¹