June 2017 NY Regents Chemistry Exam
46 What is the mass of an original 5.60-gram sample of iron-53 that remains unchanged after 25.53 minutes? (1) 0.35 g
(2) 0.70 g
(3) 1.40 g
(4) 2.80 g
Solution: Table N on the Reference Table shows radioisotopes and their half lives. A half life is an amount of time that it takes for a radioisotopes mass to become a half of the original amount. Iron-53 has a half live of 8.51 min. Let's find out how many half lives are in 25.53 minutes.
25.53 minutes/8.51 min = 3 half lives.
The original amount is 5.60 grams. After first half life, we will have 5.60/2 = 2.8grams. After second half life, there will be 2.8/2= 1.4 grams. After third half life, there will be 1.4g/2 = 0.70 grams left
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