Answer: The fraction of the rock that is still composed of potassium-40 is 0.25
Explanation:
To calculate the fraction of the rock that is still composed of K-40, we use the formula:
[tex]a=\frac{a_o}{2^n}[/tex]
where,
a = amount of reactant left after n-half lives
[tex]a_o[/tex] = Initial amount of the reactant
n = number of half lives = 2
Putting values in above equation, we get:
[tex]\frac{a}{a_o}=\frac{1}{2^2}\\\\\frac{a}{a_o}=\frac{1}{4}=0.25[/tex]
Hence, the fraction of the rock that is still composed of potassium-40 is 0.25
