Posted by
**Tracy** on
.

Okay so I have this assignment and I thought I knew how to do this but appearntly I am just lost. I know what the answer is suposed to be and I am deifantly no wheres near that. If you can sorta point me in the right direction that would be great. Thanks! The question is:

The isotope 99mTc, (t ½ = 6.01 hours) is used in the diagnostic imaging of the heart and other organs. For diagnosing heart function, a particular dose has an activity of 3.09 x 108 disintegrations per second. Calculate the mass in grams of 99mTc required to produce this activity level. Use 98.91 amu as the atomic mass of 99mTc.

I thought you would use A=landaN and solve for N. And then you woudl get landa by using landa=0.693/(t1/2) and then just divide the amount for N by avagadros number to get moles, then change the moles to grams. Then chnage the mass they give you to grams and multiply the moles that you found for N by the gram amount and you would get the mass. I don't know if that makes sense but it does on my paper.

Use Activity= lambda*N Yes, lambda is .693/t1/2. Yes, you proceeded correctly, get moles, then grams.

N/avonumber * 98.91 = mass you are looking for.

But that doesnt work out cause that's what I did. But you still need to chnage to grams, and when you do it is WAYYYY wrong. lol I'm so confused

let me see your calcs.

Okay:

A= 3.09 X10^8 dis./s

t1/2= 6.01hrs

A=landaN

N=A/landa

Units are in seconds so gotta chnage the t1/2 to seconds

Landa=(0.693/6.01hrs)(1hr/60min)(1min/60s)

=0.00003203 /s

N=(3.09 x 10^8 dis/s)/(0.00003203 /s)

= 9.647205745 x 10^12 N

9.647205745 x 10^12N(1mol/6.022 x 10^23)

= 1.601993647 x 10^-11mol

98.91amu(1.66054 x 10^-24g /1amu)

= 1.642440114 x 10^-22g

= 1.601993647 x 10^-11mol (1.642440114 x 10^-22g /1mol)

=2.63 x 10^-33g

The answer is supposed to be 1.58 x10^-9g

Okay:

A= 3.09 X10^8 dis./s

t1/2= 6.01hrs

A=landaN

N=A/landa

Units are in seconds so gotta chnage the t1/2 to seconds

Landa=(0.693/6.01hrs)(1hr/60min)(1min/60s)

=0.00003203 /s

N=(3.09 x 10^8 dis/s)/(0.00003203 /s)

= 9.647205745 x 10^12 N

9.647205745 x 10^12N(1mol/6.022 x 10^23)

= 1.601993647 x 10^-11mol
**<font=14>You are OK to here. Now just multiply 1.602 x 10^-11 x 98.91 = 1,58 x 10^-9 g**</font=12>

98.91amu(1.66054 x 10^-24g /1amu)

= 1.642440114 x 10^-22g

= 1.601993647 x 10^-11mol (1.642440114 x 10^-22g /1mol)

=2.63 x 10^-33g

The answer is supposed to be 1.58 x10^-9g

N/avonumber * 98.91 = mass you are looking for.

But N= A/lambda= A*t1/2/.693

Now t1/2 has to be in seconds...

N= 3.09E8*6.01hr*3600sec/hr /.693

N= 9.65E12

mass= N/avonumber * 98.91

=9.65E12*98.91/6.02E23

=1.59 nanograms

check that.

Ohhh okay. so I think what I was doing wrong was just the fact that I was changing the AMU to grams, then dividing it and I forgot that amu can be considered grams at some points/

Okay thanks, I think I got it. I got 2.0E-9 so at least I know I'm in the right area. Thanks sooooo much.

I went through this independently and obtained 1.58 x 10^-9 g, also.