Question

17. Nowaday, the ratio between the number of 2 isotopes 235U and 238U are: 0.72%. Half life of 235 U and 238U are 7.04x108 years; 44.7x108 years, respectively. That ratio 2 million years ago is: a) 2.33% b) 0.27% c) 0.72% d) 0.78%

Answer 1

As time duration of 1 million years is very small compared to their half-lives, there is no appreciable change in the number of isotopes.

Ratio practically remains the same.

No be the number of U-235 isotope amillion years ago Mo be the number of u-238 isotope a million years ago. NA - of 0-235 i sotope Nowaday Ne - be the ovember of 0-238 isotope nowaday. be the ocember TA= 7.oux108 yours is the U-235 half wofe of is the half life of 0-258. TB 44.7 x 108 years -at From N'=Ne for U-235, NA = Nae for - lot U-238, No = Moe eo here t= a million years. 2 x 10 years NA dat taot NA NA N8 -C2A-18) t. 42801-13m) NA NB on reananging, (2A-2B) t liven NA NB N 0.72% = 0.0072 NA Ne' ratio of isotopes U-235 and u-238 2 millon and DA = 0.693 decay constant years ago is TA z 0.693 0.693 x 2010 NA -0.0072x e 7.oux 108 uu.7x108 No 2x202 0.693 0.693 0.0072 e 7.04 44.7 (0.0984 - 0.0155) x2x10-2 0.0072 e 0.0829x2x102 = 0.0072 e 0.00166 = 0.0072 e = 0.0072 × 1.002 0.0072 0.72%