A researcher uses the helium dilution method to determine a subject's lung volume. The formula used is $V_2 = V_1 \times \frac{C_1 - C_2}{C_2}$. If the initial volume of the spirometer ($V_1$) is 2 L, the initial helium concentration ($C_1$) is 10%, and the final concentration ($C_2$) is 5%, what is the subject's lung volume ($V_2$)?
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Correct answer: 2 L
Using the law of conservation of mass: $V_2 = 2 \times \frac{10 - 5}{5} = 2 \times \frac{5}{5} = 2$ L. This method is commonly used to measure the Functional Residual Capacity (FRC).
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