Uranium-235 has a half-life of 7.04 times 10 years. How many years will it take

Question

Uranium-235 has a half-life of 7.04 times 10 years. How many years will it take for 99.9% of a U-235 sample to decay? a. 7.0 times 10^5 yr b. 1.0 times 10^6 yr c. b. 4.7 times 10^9 yr d. 4.9 times 10^9 yr e. 7.0 times 10^9 yr When the radioactive nuclide.^199At undergoes alpha emission, what is the product nuclide? a.^197Bi b.^203Fr c.^197Rn d. 195Bi e.^199 Fr If a tree dies and the trunk remains undisturbed for 14996 years, what percentage of original^14C is still present? (half life of^14C-5730 years) a. 32.0% b. 24.0% c. 83.0% d. 16.0% An atom with 45 protons has a mass number of 100. It must contain how many neutrons? a. 145 b. 45 c. 100 d. 55 e. none of these

Explanation / Answer

Ans 52. E. 7.2 x 109 years

Half life period is the time required by a sample to reduce to half of its original value,

Nt = No (1/2)t/t 0.5

where Nt is sample left after time t which is 0.1% here

No is the initial sample taken ( let it be 100)

t is time lapsed

ans t 0.5 is the half life period of sample which is 7.04 x108 years

so substituting the values in equation above,

0.1 = 100 (1/2)t/ 7.04 x10^8

t = 7.0 x 109 years

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