ADA problem 2009 Gen chem

[sexysummer]

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What percent of the initial radioactivity will remain after one hour has passed if the half-life of a radioactive isotope is 20 minutes?

This is from the ADA 2009 practice test. I am really rusty on radioactivity problem and do not know how to set them up? Please help me.

 

[Lnguyen7]

The half-life is 20 minutes, so one hour is 3 half-lives. Initially you have 100% of the sample, after 3 half-lives, you will go from 100 to 50 to 25 to 12.5%. Thus 12.5% of the initial sample remains.

 

[sexysummer]

The half-life is 20 minutes, so one hour is 3 half-lives. Initially you have 100% of the sample, after 3 half-lives, you will go from 100 to 50 to 25 to 12.5%. Thus 12.5% of the initial sample remains.

OK, thanks for replying.

another Question.

Why is it choice B? Is ideal gas @ STP 0 Celsius and 1 atm?

Thanks

 

[sexysummer]

The half-life is 20 minutes, so one hour is 3 half-lives. Initially you have 100% of the sample, after 3 half-lives, you will go from 100 to 50 to 25 to 12.5%. Thus 12.5% of the initial sample remains.

How to solve this one?

 

[Lnguyen7]

OK, thanks for replying.

another Question.

View attachment 196550

Why is it choice B? Is ideal gas @ STP 0 Celsius and 1 atm?

Thanks

Yes, STP is 0 Celcius and 1atm. The problem however does not say anything about STP, only the volume of the ideal gas is zero. You cannot assume that it is STP for any ideal gas. For ideal gas PV = nRT here. You see that if V is 0, then T has to be 0 KELVIN, or -273C.

 

[sexysummer]

The half-life is 20 minutes, so one hour is 3 half-lives. Initially you have 100% of the sample, after 3 half-lives, you will go from 100 to 50 to 25 to 12.5%. Thus 12.5% of the initial sample remains.

Hello,
Thanks. I sent you another question from gen Chem. Could you assist me with those if possible?

Thank you sir.

 

[sexysummer]

Yes, STP is 0 Celcius and 1atm. The problem however does not say anything about STP, only the volume of the ideal gas is zero. You cannot assume that it is STP for any ideal gas. For ideal gas PV = nRT here. You see that if V is 0, then T has to be 0 KELVIN, or -273C.

Thank you so much. What about the radioactivity Problem under the first problem?

 

[sexysummer]

Yes, STP is 0 Celcius and 1atm. The problem however does not say anything about STP, only the volume of the ideal gas is zero. You cannot assume that it is STP for any ideal gas. For ideal gas PV = nRT here. You see that if V is 0, then T has to be 0 KELVIN, or -273C.

Hi,
I want to send you a private message on sdn? How?

 

[Lnguyen7]

View attachment 196553

How to solve this one?

The half-life is 10.2 mins. The time is 51 mins which is 5 half-lives. The initial concentration is 1 mol/L, so after 5 half-lives, you go from 1-> 1/2 -> 1/4 -> 1/8 -> 1/16 -> 1/32. So 1/32 remaining.

I'm not sure how to send private message on SDN.

 

[Dentoni]

Hi, Here is how you send private message: first click on the username of the person you want to send a message to and something will show up, then you click on "Start conversation" .