# Calculating decay constants and half-life

by Steph191290
Tags: constants, decay, halflife
 P: 30 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. 2. Relevant equations decay constant = a/N n=m/Mr 3. 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, im not sure wher to go from here any help would be appreciated.
 Emeritus Sci Advisor PF Gold P: 9,789 You might want to check the value of Avogadro's Constant
 P: 30 oops that should have been 10^23 lol, thanks i'll try that x
P: 30

## Calculating decay constants and half-life

now i get 1.4448x10^15 which is still wrong, im so confused.
x
Emeritus
PF Gold
P: 9,789
 Quote by Steph191290 now i get 1.4448x10^15 which is still wrong, im 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?
 P: 30 9.03x10^15 x
Emeritus
PF Gold
P: 9,789
 Quote by Steph191290 9.03x10^15 x
Looks right to me.
 P: 30 well i dont know what i did but i have just gotten the right answer thanks lol xx
Emeritus