Question

235U decays to 207Pb. 1. Which is the parent, U or Pb? 2. If 3 half lives have passed, what is the parent daughter ratio? a) 50/50 b) 25/75 c) 12.5/87.5 d) 6.25/93.75 e) 3.125/96.875 3. The half life for 235U decay to 207Pb is 704 Ma. How old is the rock in the previous question? The half-life of 40K is 1.3 billion years. 4. If a 3.9 billion year old rock contains 100g of 40K, how many grams of 40K did the rock contain when it first formed? a. 400g b. 600g c. 12.5g d. 200g e. 800g

Answer 1

1.parent compound is that compound which decays into a daughter compound hence here the parent compound is uranium. (u)

2. We know N= N_{​​​​​​I} /(2)ⁿ

Where N= amount of substance left

N_{​​​​​​i}= initial amount of substance and n= no of half lives

So when 3 half lives passed n= 3

So N= N/ (2)³= N/8

Hence amount of daughter nuclei formed= N- N/8= 7N/8

So ratio is N/8. Divided by 7N/8= 1/7 = 12.5/87.5

2222222 til 704 Ma We know, iMac 10 years. til 704 x 10 years. Also, To = - tila xn 704 x 106 x 3 2112x 106 years or 2 112 Ma til 1-3 billion year T = 3.9 billion year. We know, TE nx ti/2 no I a 3.9 4) 3 til 13 And N = No ) No z> No Given) No 100g -No:7 d No 2 8 x N:) 8x 100 2 800g (e) option is correct