Half Life

[Dencology]

i thought the half life would be 36, but it is 48, how?

1. If the half-life of an isotopic nuclide is 24 hours, how long will it take for 75% of the sample to decay?

0 1/2 life ------------>100% 0 hrs
1 1/2 life 50% 24hrs
2 1/4 life 75% 36 hrs

guys what is the best way to approach this problem?


2. How many milliliters of 0.05 M HCl are required to turn 20 ml of 0.05 M KOH into a solution of pH 2.00?

so if to neutralize this solution we need 20 ml of HCl. now to get a pH of 2 we need more that 20 ml. how do we do this?
(please show your work)

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[Dencology]

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[Orgodox]

You set it up right and were on the right track but im not sure why you rote 24 hours for the first half life then 36 for the next. After 24 hours thats one half life you have half the samle. After another 24 hours you only have 1/4 left so 75% decayed but that took 48 hours 2 full half lives.

 

[Hugh Mannity]

Remember that the half life is not a linear function. In one half life cycle, half of the compound decays. In 24 hours, 50% decays, in the next 24 hours, 50% of the remaining sample decays, leaving you with 25%.

 

[Mstoothlady2012]

i thought the half life would be 36, but it is 48, how?

1. If the half-life of an isotopic nuclide is 24 hours, how long will it take for 75% of the sample to decay?

0 1/2 life ------------>100% 0 hrs
1 1/2 life 50% 24hrs
2 1/4 life 75% 36 hrs

guys what is the best way to approach this problem?


2. How many milliliters of 0.05 M HCl are required to turn 20 ml of 0.05 M KOH into a solution of pH 2.00?

so if to neutralize this solution we need 20 ml of HCl. now to get a pH of 2 we need more that 20 ml. how do we do this?
(please show your work)

you are not suppose to half the half life everytime you decay half the sample...makes sense?

 

[PooyaH]

i thought the half life would be 36, but it is 48, how?

1. If the half-life of an isotopic nuclide is 24 hours, how long will it take for 75% of the sample to decay?

0 1/2 life ------------>100% 0 hrs
1 1/2 life 50% 24hrs
2 1/4 life 75% 36 hrs

just use this format: 1/2^n, n being the number of half life! so 2 half life would give you 1/4 (25%) which means 75% of your sample would decay!!! And since the half life is every 24 hours, 2 half life would be 48 hours!

 

[PooyaH]

you are not suppose to half the half life everytime you decay half the sample...makes sense?

I don't get what you mean but after every half life you get half of what you have!!! ex. 100-->50-->25-->12.5-->etc...

 

[Mstoothlady2012]

I don't get what you mean but after every half life you get half of what you have!!! ex. 100-->50-->25-->12.5-->etc...

yes you are right. But the OP is also reducing the half life by half every time the sample decays by half. Half life stays constant.

 

[PooyaH]

yes you are right. But the OP is also reducing the half life by half every time the sample decays by half. Half life stays constant.

Oh now I gotcha! lol Sorry!