The amount of oxygen dissolved in a sample of water can be determined by using thallium...

The amount of oxygen dissolved in a sample of water can be determined by using thallium metal containing a small amount of the isotope Tl- 204. When excess thallium is added to oxygen-containing water, the following reaction occurs.

After reaction, the activity of a 25.0-mL water sample is 745 counts per minute (cpm), caused by the presence of Tl⁺-204 ions. The activity of Tl-204 is 5.53 × 10⁵ cpm per gram of thallium metal. Assuming that O₂ is the limiting reactant in the above equation, calculate its concentration in moles per liter.