The curie (Ci) is a commonly used unit for measuring nuclear radioactivity: 1 cu

Question

The curie (Ci) is a commonly used unit for measuring nuclear radioactivity: 1 curie of radiation is equal to 3.7 x 10^10 decay events per second (the number of decay events from 1 g radium in 1 s). A 1.7-mL sample of water containing tritium was injected into a 170-lb person. The total activity of radiation injected was 86.5 mCi. After some time to allow the tritium activity to equally distribute throughout the body, a sample of blood plasma containing 2.0 mL water at an activity of 3.6 µCi was removed. From these data, calculate the mass percent of water in this 170-lb person.

Mass percent equals?

Explanation / Answer

Let the water in the body is , x L

Half life of tritium (12.32 years) is large compared to the time considered in this question

so, we can assume that activity directly depends upon no. of moles

so, activity per liter = 3.6*10^-6/(2/1000) = 1.8*10^-3

so, total activity = 1.8*10^-3*x

which will be equal to entered activity = 86.5*10^-3

so, x = 86.5*10^-3/(1.8*10^-3) = 48.056 L

1 L water = 2.2 lbs

so % of water = 2.2*48.056*100/170 = 62.19 %

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