Question 25

Question 25: The half-life of a radioactive element is the time it takes for half of the substance to decay. Given that the half-life of element X is 5 days, after 10 days (which is 2 half-lives), we can calculate the remaining amount of the sample as follows:

1. After the first half-life (5 days):
   \(100 \text{ g} \rightarrow 50 \text{ g}\)

2. After the second half-life (10 days):
   \(50 \text{ g} \rightarrow 25 \text{ g}\)

So, the correct answer is c. 25 g.

Question 26: When the temperature of a gas decreases, the volume also typically decreases. This behavior is described by Charles's Law, which states that the volume of a given mass of gas is directly proportional to its temperature (in Kelvin) when the pressure is held constant.

So, the correct answer is b. The volume decreases.