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

160 g

Explanation:

To calculate we use the formula:

where,

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

= Initial amount of the reactant = ?

n = number of half lives  = 3

Putting values in above equation, we get:

hloride, what is the mass of calcium in

explanation:

since electrons are negative the answer is neagtaive