Posted by **Trixie** on Friday, April 16, 2010 at 6:35pm.

Q1. The blood volume in a cancer patient was measured by injecting 5.0 mL of Na2SO4(aq) labeled with 35S (t1/2 = 87.4 d). The activity of the sample was 300 µCi. After 22 min, 12.9 mL of blood was withdrawn from the man and the activity of that sample was found to be 0.75 µCi. Report the blood volume of the patient.

I listed below already about where I am confused....I have been doing these problems by converting the t1/2 into minutes and plugging everything in to N=Ne^-kt....but I think I am confusing some variables because my answers are a bit off... Same applies for Q2.

A radioactive sample contains 3.25 1018 atoms of a nuclide that decays at a rate of 3.4 1013 disintegrations per 26 min.

(a) What percentage of the nuclide will have decayed after 159 d?

%

(b) How many atoms of the nuclide will remain in the sample?

atoms

(c) What is the half-life of the nuclide?

days

I don't need the answers handed to me...I just want to know how to do it. As before, I listed my work already....I don't know if I am making mistakes in converting or if I need to use N=Ne^-kt in a different way than I already described. Thanks for the help...

- Chemistry -
**Trixie**, Friday, April 16, 2010 at 6:43pm
I figured out the first one...still struggling with the 2nd...

- Chemistry -
**bobpursley**, Friday, April 16, 2010 at 8:58pm
Amountleft= Originalamount*e^-kt

-Rate=-k*original amount*e^-kt

3.4E13/26=3.25E18(k)e^k0

k= 4.023E-7 /min or 5.74E-4/day

amount left/original amount= e^-kt

= e^(-5.74E-4 *159.21E-2)

fraction left=9.21E-1=.921

fraction decayed=.079

percent decayed=7.9

halflife:

5.74E-4= ln2/thalflife

thalfflife= 1207 days

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