1.Tritium, hydrogen-3, is formed in the upper atmosphere by cosmic rays, similar
ID: 635273 • Letter: 1
Question
1.Tritium, hydrogen-3, is formed in the upper atmosphere by cosmic rays, similar to the formation of carbon-14. Tritium has been used to determine the age of wines. A certain wine that has been aged in a bottle has a tritium content only 72% of that in a similar wine of the same mass that has just been bottled. How long has the aged wine been in the bottle? Half –life of tritium is 12.3y
a) 5.8y b) 6.7y c) 8.9y d) 17.1y e) 22.6y
2. Uranium-238 decays to lead-206 with a half-life of 4.5 x 109y. If a rock from Australia contains 0.438g of 206Pb and 1.00g 238U, how old is the rock. Assume that the rock did not contain any 206Pb when it was formed.
a) 2.7 x 109y b) 4.1 x 109y c) 5.4 x 109y d) 13.5 x 109y e) 43.8 x 109y
3. A jawbone was found at the archaeological site at Folsom, New Mexico. The activity of the carbon in the jawbone was 4.5 disintegrations per minute per gram. What was the age of the jawbone? Carbon from living material gives 15.3 disintegrations per minute per gram and the half life of 14C is 5730 y.
a) 500y b) 1,000y c) 5,000y d) 10,000y e) 15,000y
4. A 500mg sample of rock was found to have 2.45 x 10-6 mol 40K (t1/2 = 1.3 x 109y) and 1.23 x 10-7mol 40Ar. Assuming that the 40Ar accounts for 10% of the 40K which has decayed (90% ended up as 40Ca), how old is the rock?
a) 8.7 x 106y b)2.1 x 107y c) 7.8 x 108y d) 1.3 x 109y e) 6 .5 x 109y
Explanation / Answer
Ans 1
Given data
Let the initial amount = 100
Final amount remain = 72
Final amount = Initial amount x (0.5)^(time/half life)
72 = 100 x (0.5)^(time/12.3)
0.72 = (0.5)^(time/12.3)
Take natural logarithm on both sides
ln 0.72 = (time/12.3) ln(0.5)
(time/12.3) = (ln 0.72) / (ln 0.5) = 0.4739
Time = 12.3*0.4739 = 5.8y
Option A is the correct answer
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