Question

iodine-131 has a half-life of 8 days. how much of a 1000 mg sample would be left after 24 days?
250 mg
500 mg
125 mg
62.5 mg
question 5 (0.0862 points)
which of the following measurements has the correct significant figures for a graduated cylinder with marks to the ml?
52 ml
52.1 ml
52.12 ml
50 ml

Explanation:

First Question (Iodine - 131 Half - Life)

Step1: Determine number of half - lives

The half - life of Iodine - 131 is 8 days. The time elapsed is 24 days. To find the number of half - lives ($n$), we use the formula $n=\frac{\text{total time}}{\text{half - life}}$. So, $n = \frac{24}{8}=3$.

Step2: Calculate remaining amount

The formula for the amount of a substance remaining after $n$ half - lives is $A = A_0\times(\frac{1}{2})^n$, where $A_0$ is the initial amount. Here, $A_0 = 1000$ mg and $n = 3$. So, $A=1000\times(\frac{1}{2})^3=1000\times\frac{1}{8}=125$ mg.

A graduated cylinder with marks to the mL (whole number of mL) can be read to one decimal place (the estimated digit). So, we can estimate to the tenths place.

• 52 mL: This is to the whole number, we can estimate to one decimal place, so this is not the most precise.
• 52.1 mL: This is to the tenths place, which is appropriate for a graduated cylinder marked to the mL (we can estimate the tenths place).
• 52.12 mL: This is to the hundredths place, but a graduated cylinder marked to the mL can't be read to the hundredths place as we can only estimate one digit beyond the markings.
• 50 mL: This is a less precise measurement compared to 52.1 mL.

Answer:

125 mg

Second Question (Significant Figures for Graduated Cylinder)