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It takes 180 minutes for a 200 -mg sample of an unknown radioactive substance to decay to \(112 \mathrm{mg}\). What is the halflife of this substance?

Short Answer

Expert verified
The half-life of the unknown radioactive substance is approximately \(324.84\) minutes.

Step by step solution

01

Write down the decay formula to be used

The radioactive decay formula is given by: \[ N(t) = N_{0} \left(\frac{1}{2}\right)^\frac{t}{t_{1/2}} \] Where \(N(t)\) is the remaining amount of the substance after time \(t\), \(N_{0}\) is the initial amount of the substance, \(t_{1/2}\) is the half-life, and \(t\) is the time of decay.
02

Plug in the given information into the decay formula

We are given that the initial mass (\(N_{0}\)) is 200 mg, the final mass (\(N(t)\)) is 112 mg, and the decay time (\(t\)) is 180 minutes. We need to find the half-life (\(t_{1/2}\)). Plug these values into the decay formula: \[ 112 = 200 \left(\frac{1}{2}\right)^\frac{180}{t_{1/2}} \]
03

Solve for t_{1/2}

To find \(t_{1/2}\), we will first divide both sides of the equation by 200: \[ \frac{112}{200} = \left(\frac{1}{2}\right)^\frac{180}{t_{1/2}} \] Next, take the logarithm base \( \frac{1}{2} \) of both sides: \[ \log_{1/2}\left(\frac{112}{200}\right) = \frac{180}{t_{1/2}} \] Now, multiply both sides by \(t_{1/2}\) and divide both sides by \(\log_{1/2}\left(\frac{112}{200}\right)\) to isolate the half-life: \[ t_{1/2} = \frac{180}{\log_{1/2}\left(\frac{112}{200}\right)} \] Finally, use a calculator to evaluate the expression, which gives: \[ t_{1/2} \approx 324.84 \: minutes \] The half-life of the unknown radioactive substance is approximately 324.84 minutes.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Half-Life Calculation
The concept of half-life is crucial in understanding radioactive decay. Half-life refers to the time required for half of a given amount of a radioactive substance to decay. This doesn't mean the substance vanishes, but its radioactive nature reduces. Knowing the half-life helps us predict how long a substance will remain active.
To calculate half-life, we often use the decay formula, which involves the initial quantity of the substance, the remaining quantity after a certain time, and the decay time. The formula applied in this context is:\[ N(t) = N_{0} \left(\frac{1}{2}\right)^\frac{t}{t_{1/2}} \]In this equation, \(N(t)\) is what remains of the substance after time \(t\), \(N_{0}\) is the initial amount, and \(t_{1/2}\) is the half-life we need to determine. It's a systematic way to predict the decrease of radioactive material over time.
Understanding half-life calculations helps in diverse applications such as dating archaeological findings or managing nuclear waste.
Decay Formula
Radioactive decay follows a predictable pattern usually represented mathematically through a decay formula. This equation defines how a radioactive substance diminishes over time. It is a fundamental concept in nuclear physics and chemistry for evaluating how substances transform.
The decay formula used is:\[ N(t) = N_{0} \left(\frac{1}{2}\right)^\frac{t}{t_{1/2}} \]Here’s what each term means:
  • \(N(t)\): The remaining amount of the substance after time \(t\).
  • \(N_{0}\): The initial amount of the substance at the beginning.
  • \(t\): The time elapsed during the decay.
  • \(t_{1/2}\): The half-life, which is the time it takes for the quantity to reduce by half.
This formula is derived from the basic principles of exponential decay, where decay happens at a rate proportional to the current amount of the substance. This ensures that the evaluation of decay is accurate and consistent across different scenarios.
Radioactive Substances
Radioactive substances contain unstable nuclei that lose energy by emitting radiation in a process known as radioactive decay. These substances can vary greatly but commonly include isotopes used in medical applications, energy production, and scientific research. Over time, these isotopes transform into more stable forms, releasing particles and energy in the process.
Understanding the behavior of radioactive substances is paramount, particularly in how they decay. This decay spans predictable time frames governed by their respective half-lives, allowing for controlled uses in environments like nuclear reactors or medical labs.
Commonly used radioactive substances include:
  • Uranium-238: Used in nuclear reactors and weapons.
  • Carbon-14: Used in radiocarbon dating.
  • Iodine-131: Applied in medical diagnostics and treatment.
Using radioactive substances safely involves understanding their half-lives and decay processes, allowing scientists and professionals to harness their energy effectively while minimizing risks.

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Most popular questions from this chapter

A \(2.5-\mathrm{mL}\) sample of \(0.188 \mathrm{M}\) silver nitrate solution was mixed with \(2.5 \mathrm{~mL}\) of \(0.188 \mathrm{M}\) sodium chloride solution labeled with radioactive chlorine-36. The activity of the initial sodium chloride solution was \(2.46 \times 10^{6} \mathrm{~Bq} / \mathrm{mL}\). After the resultant precipitate was removed by filtration, the remaining filtrate was found to have an activity of 175 Bq/mL. (a) Write a balanced chemical equation for the reaction that occurred. (b) Calculate the \(K_{s p}\) for the precipitate under the conditions of the experiment.

Which of the following statements about the uranium used in nuclear reactors is or are true? (i) Natural uranium has too little \({ }^{235} \mathrm{U}\) to be used as a fuel. (ii) \({ }^{238} \mathrm{U}\) cannot be used as a fuel because it forms a supercritical mass too easily. (iii) To be used as fuel, uranium must be enriched so that it is more than \(50 \%^{235} \mathrm{U}\) in composition. (iv) The neutron-induced fission of \({ }^{235} \mathrm{U}\) releases more neutrons per nucleus than fission of \({ }^{238} \mathrm{U}\)

What do these symbols stand for? (a) \({ }_{0}^{0} \gamma,(\mathbf{b}){ }_{2}^{4} \mathrm{He},\) (c) \({ }_{0}^{1} \mathrm{n} .\)

Phosphorus- 32 is commonly used in nuclear medicine for the identification of malignant tumors. It decays to sulphur- 32 with a half-life of 14.29 days. If a patient is given 3.5 mg of phosphorus-32, how much phosphorus-32 will remain after 1 month (i.e. 30 days)?

Each of the following nuclei undergoes either beta decay or positron emission. Predict the type of emission for each: (a) \({ }_{38}^{90} \mathrm{Sr}\) (b) \({ }_{38}^{85} \mathrm{Sr}\) (d) sulfur-30. (c) potassium- 40 ,

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