Posted by **Mary** on Saturday, February 9, 2008 at 7:49pm.

A Cesium 137 sample has a measured radioactivity of 50 microcuries.Cesium 137 has a half-life of 30 years.

How long will it take this sample to be reduced to a residual radioactivity of 6.25 microcuries?

Is there a formula to calculate half life?

- Chemistry please help! -
**DrBob222**, Saturday, February 9, 2008 at 9:01pm
k = 0.693/t_{1/2}

Then ln(No/N) = kt

No = 50 microcuries

N = 6.25 microcuries

k from above (t1/2 = 30 years).

t = solve for this.

- Chemistry please help! -
**Anonymous**, Wednesday, February 25, 2015 at 2:44pm
A 45 gram sample of a substance that's used to preserve fruit and vegetables has a k-value of 0.1109.

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