Half-Life quiz #1 Flashcards
Half-Life quiz #1
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What fraction of a reactant remains after two half-lives have passed?
After two half-lives, 1/4 (or 25%) of the original reactant remains. What fraction of a reactant has been consumed after 4 half-lives in a first-order reaction?
After 4 half-lives, 15/16 of the reactant has been consumed. Which definition describes a half-life?
Half-life is the time required for half of a substance to decay or decompose. What percentage of daughter isotopes would be present after 5 half-lives have passed for 87Rb?
After 5 half-lives, 96.875% of the original sample has decayed, so 96.875% daughter isotopes are present. What is the rate constant of a first-order reaction if the half-life is 2.50 min?
k = ln(2) / 2.50 min ≈ 0.277 min⁻¹. What percentage of a substance remains after 7 half-lives have passed?
After 7 half-lives, 0.78% of the original substance remains. Sodium-24 has a half-life of 15 hours. How many half-lives will pass in 5 days?
5 days = 120 hours; 120 hours / 15 hours = 8 half-lives. Which statement below regarding the half-life of a zeroth-order reaction is true?
The half-life of a zeroth-order reaction depends on the initial concentration and decreases as concentration decreases. How many grams of cobalt-60 will remain after three half-lives?
After three half-lives, 1/8 of the original amount remains. A first-order reaction is X% complete at the end of Y minutes. What is the value of the rate constant k?
Use the formula: fraction remaining = e^(−kt); solve for k using the given percentage and time. What remains after a sample undergoes two half-lives?
After two half-lives, 1/4 of the original sample remains; 3/4 has decayed into daughter products. What is the half-life of radon-220?
The half-life of radon-220 is a specific value found in reference tables; it is approximately 55.6 seconds. Which of the following statements about the half-life of a radioactive isotope is true?
The half-life of a radioactive isotope is constant and independent of the initial amount. Which is the correct formula for calculating the age of a meteorite using half-life?
Age = (number of half-lives) × (half-life duration). What is the half-life of a radioactive isotope?
The half-life is the time required for half of the radioactive isotope to decay. What occurs during one half-life?
During one half-life, half of the original substance decays or decomposes. What is the half-life of this isotope?
The half-life is the time it takes for half of the isotope to decay; the specific value depends on the isotope. What is a half-life?
A half-life is the time required for half of a substance to decay or decompose. A sample is in the second half-life. What percent of the sample is stable at this point?
After two half-lives, 75% of the original sample has decayed (is stable if referring to daughter product). What fraction of a sample’s radioactive atoms remain after four half-lives?
After four half-lives, 1/16 of the radioactive atoms remain. How much of a sample remains after three half-lives have occurred?
After three half-lives, 1/8 of the original sample remains. Which information can be determined using half-life?
Half-life can be used to determine the time required for a substance to decay to a certain fraction and the age of samples. Which statement accurately describes a half-life?
A half-life is the time required for half of a substance to decay, and for first-order reactions, it is constant. What is the half-life for a particular reaction if the rate law is rate = (1301 min⁻¹)[A]?
For a first-order reaction, half-life = ln(2) / k = 0.693 / 1301 min⁻¹ ≈ 0.000533 min. Seaborgium-266 has a half-life of 30 seconds. How many half-lives will pass in 30 minutes?
30 minutes = 1800 seconds; 1800 / 30 = 60 half-lives. If the parent isotope has a half-life that equals 1.75 million years, how long will it take for half of the sample to decay?
It will take 1.75 million years for half of the sample to decay. If 87.5 percent of a sample of pure 131I decays in 24 days, how many half-lives have passed?
87.5% decay means 12.5% remains; this is 3 half-lives. The half-life of bromine-74 is 25 min. How much of a 4.0 mg sample is still active after 75 min?
75 min is 3 half-lives; 4.0 mg × (1/8) = 0.5 mg remains. Which time frame correctly describes the range of half-lives?
Half-lives can range from fractions of a second to billions of years, depending on the substance. Iodine-131 has a half-life of 8.0 minutes. How many half-lives will pass in 120 minutes?
120 / 8.0 = 15 half-lives. The decay constant for the element X is 6.931 yr⁻¹. What is the half-life?
Half-life = ln(2) / k = 0.693 / 6.931 yr⁻¹ ≈ 0.1 years. If you start with 80 radioactive atoms, how many daughter atoms exist after 2 half-lives?
After 2 half-lives, 60 atoms have decayed into daughter atoms. The cadmium isotope 109Cd has a half-life of 462 days. A sample begins with 1.0×10¹² 109Cd atoms. How many atoms remain after two half-lives?
After two half-lives, 1/4 of the original atoms remain: 2.5×10¹¹ atoms.
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