Calculating decay constants and half-life

[Steph191290]

A scientist wishes to find the age of a sample of rock. Realising that it contains radioactive potassium, which decays to give a stable form of argon, the scientist started by making the following measurments:

decay rate of the potassium in the sample = 0.16Bq
mass of potassium in the sample = 0.6x10^-6g
mass of argon in the sample = 4.2x10^-6g

The Molar mass of the potassium is 40g. show that the decay constant for potassium is 1.8x10-17 and its half-life is 1.2x10^9years.

Homework Equations

decay constant = a/N
n=m/Mr

The Attempt at a Solution

N=m/Mr
N=0.6x10^-6/40
N=1.5x10^-8

N=1.5x10^-8 x 6.02x10^-8
N=9.03 x 10^-16

decay constant = a/N
decay constant = 0.16/9.03x10^-16

I got this far then got stck as the answer was wrong, I am not sure wher to go from here any help would be appreciated.

 

[Hootenanny]

You might want to check the value of Avogadro's Constant

 

[Steph191290]

oops that should have been 10^23 lol, thanks i'll try that
x

 

[Steph191290]

now i get 1.4448x10^15 which is still wrong, I am so confused.
x

 

[Hootenanny]

now i get 1.4448x10^15 which is still wrong, I am so confused.
x

Your method is correct, you must just be punching the numbers into your calculator wrong. How many K atoms did you calculate?

 

[Steph191290]

9.03x10^15
x

 

[Hootenanny]

9.03x10^15
x

Looks right to me.

 

[Steph191290]

well i don't know what i did but i have just gotten the right answer thanks lol
xx

 

[Hootenanny]

well i don't know what i did but i have just gotten the right answer thanks lol
xx

As I said above, your method was spot on but you were probably just hitting the wrong buttons on your calculator.

 

[Steph191290]

thanks lol, i do that a lot it seems, better brush up on those skills before the exam lol.