# Potassium 40 dating formula

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Therefore, the amount of argon-40 produced by the decay of potassium-40 is 11% of the total mass of potassium that has decayed: 4.2 x 10 kg of argon-40 produced in the Earth's crust Argon is a common gas, making up 0.9% by volume or 1.3% by mass of the atmosphere.Only nitrogen and oxygen (and sometimes water vapor, depending on the humidity) are more plentiful in the atmosphere.Potassium-40 decays to calcium-40 89% of the time and to argon-40 11% of the time.Therefore, the quantity of potassium-40 that decayed to produced the argon is: (X micrograms potassium-40) x 0.11 = (6.0 micrograms argon-40) X = 6.0/0.11 = 55 micrograms potassium-40 decayed to produce argon-40 55 micrograms of potassium-40 decayed away and 120 micrograms remain in the rock.Thus, the crustal abundance of potassium-40 is: 0.012 x 0.00012 = 1.4 x 10] = 19 parts per million potassium-40 when the Earth formed Shortly after the Earth formed, the abundance of potassium-40 was 19 parts per million. That means that 17.6 parts per million of the crust, in the form of potassium-40, has decayed away over the past 4.5 billion years.The total decayed mass is equal to the total mass of the crust multiplied by 17.6 parts per million: (2.4 x 10 kg of potassium-40 decayed Potassium-40 decays to calcium-40 89% of the time and argon-40 11% of the time.  By measuring the amount of potassium and argon in a rock sample, the age of the rock since it solidified can be determined.Almost all atmospheric argon (99.6%) is argon-40, whereas the argon in the Sun and stars, produced by stellar nucleosynthesis, is mostly argon-36.This suggests that primordial argon is in the form of argon-36 and essentially all of the argon in the atmosphere was produced by the decay of potassium-40 to argon-40.Your answers might be slightly different due to rounding.The decay of potassium-40 to argon-40 explains why there is so much argon in the atmosphere, compared with the other noble gases.