u began w/ 10g of a radioactive parent isotope how many grams of parent isotope is present after 2 half-lives?

Answer 1

You keep asking the same question in different ways. Don't you understand what a 'half life' is?
Whatever the parent isotope. a half life will lose half of the substance of the material. So for 2 half lives, you divide 10 g by two, twice. It really isn't hard. Give yourself a chance to learn. Don't depend on the kindness of strangers. That's dangerous.

Answer 2

The formula for amount remaining after n half lives is :
n=N / (2 ^ number of half lives)

So in this case it is

n= 10/(2^2)
= 10/4
= 2.5 grams

The formula mentioned above is also applicable to non integral half lives (like 1.5)

Answer 3

you might find this interactive site helpful:
http://www.colorado.edu/physics/2000/isotopes/radioactive_decay3.html

or this one:
http://www.webelements.com
Select an element from the periodic table. Go to the Nuclear Properties section in the left-hand margin and select Radioisotopes. Students are presented with all of the isotopes of that element, as well as the mass, half-life, and mode of decay of each, including references to further information such as Table of the Nuclides.

Answer 4

After a half life 50% remains, after two, 25% remains.

Hence 10g will have depleted to 10 * 0.25 = 2.5 g

Answer 5

2.5g after 2 half lives

Answer 6

2.5 grams.