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After three half-lives, a sample contains 20 grams of parent isotopes. How many grams of the parent isotope were present to start?

Respuesta :

PhyCS
Three half lives meant that the mass was halved three times. Therefore, 20 * 2 * 2 * 2 to reverse the process yields 160 grams in the beginning.

Answer: 160 g

Explanation:

To calculate we use the formula:

[tex]a=\frac{a_o}{2^n}[/tex]

where,

a = amount of reactant left after n-half lives = 20 g

[tex]a_o[/tex] = Initial amount of the reactant = ?

n = number of half lives  = 3

Putting values in above equation, we get:

[tex]20g=\frac{a_0}{2^3}[/tex]

[tex]20g=\frac{a_0}{8}[/tex]

[tex]a_o=8\times 20g=160g[/tex]

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