Need help w/ CHEM hw...regarding half-lives?

Question

THE QUESTION:

If the passing of five half-lives leaves 25.0mg of (superscript)90Sr, how much was present in the beginning?


THE EQUATION:
#Left = #Original * (1/2) [exponent]T/h
T= time
H= half life



i don't know how to solve this problem because it doesn't give info for the time OR the half life of Sr...what the heck does 'five half-lives' mean?

please help...thanks

THANK YOU everyone... i get it now

Answer 1

Well, you don't need the time of how long a half-life is, really. You know that in the time a half life goes through, the amount of whatever the thingie is is cut in half. So if it went through five half lifes, it's 1/2^5 of how much it originally was, or 1/32.

1/32 of the original amount = 25 mg
So there was 800 mg present in the beginning. I'll leave converting it into an equation to yourself (partially because I don't really understand the variables and thingies that u've put up there o.o)

Answer 2

Meg,

You are trying to solve for the amount of the original since you know what was left.

Original *(1/2)^(t/h)= Left

multiply both sides by (2)^(t/h)


Original *(1/2)^(t/h)* (2)^(t/h)= Left* (2)^(t/h)

Original=Left *2^(t/h)

Your problem says 5 half-lives therefore t=5h.

If you now plug 5h for t into the equation, you should get something that looks like:

Original = (25mg)*2^5

Good luck

Additional:

Since everyone has already told you the answer,

Original = 25*32 ------ (32 being 2^5)
= 800

Answer 3

ok a half life is losing half of its complete amount in some time

so in this problem time is irrelevant

5 half lives have passed and now you have 25 mg of Sr so that means multiply the number by 2 5 times

so 25 x 2 = 50

50 x 2 = 100

100 x 2 = 200

200 x 2 = 400

400 x 2 = 800

so it started with 800 mg

Answer 4

The equation grow to be possibly something like a = A*2^(-t/T), the place a is the quantity at time t, A is the preliminary quantity, t is the elapsed time, and T is the 0.5-existence. as a consequence, you have been given a = a million ng and A = 10 mg, as properly as T = 5 hr, and you're searching for t. you purely could desire to divide the two aspects by using A and then take the log base 2 to get -t/T equivalent to a form that could desire to be unfavourable. Then multiply the two aspects by using T to get your very final answer. formerly you initiate, you are able to desire to transform the two hundreds to kg, or another metric unit, as long as this is the same one.

Answer 5

Half-life is the time it takes for half of the substance to degrade. To get to 25.0mg, five half-lives have passed. Double the 25.0mg five times to get the original quantity. You can use your fingers to keep track of your doubling.
I get 800mg.