Question

In order to determine the volume of blood in an animal without killing it, a 1.00 mL sample of an aqueous solution containing tritium is injected into the animal's blood-stream. The sample injected has an activity of $1.8\times {10}^6$ cps(counts per second). After sufficient time for the sample to be completely mixed with the animal's blood due to normal blood circulation, 2.00 mL of blood are withdrawn from an animal, and the activity of the blood sample withdrawn is found to be $1.2\times {10}^4$ cps. Calculate the volume of animal's blood?

• A. $300$ mL
• B. $100$ mL
• C. $200$ mL
• D. None of these

Answer 1

The correct option is A $300$ mL

Let volume of the blood in the animal $=VmL$
Total activity before mixing in $=1.8\times {10}^6$ cps
Total activity after mixing in $VmL=\frac{1.2\times {10}^4}{2}\times V$
$=0.6\times {10}^4$ V cps
Thus, $0.6\times {10}^{4}V=1.8\times {10}^6$ (assuming activity remians constant)
$V=\frac{1.8\times {10}^6}{0.6\times {10}^4}=300$ mL