Answer:
2.1x10⁹ years
Explanation:
U-238 is a radioactive substance, which decays in radioactive particles. It means that this substance will lose mass, and will form another compound, the Pb-206.
The time need for a compound loses half of its mass is called half-life, and knowing the initial mass (mi) and the final mass (m) the number of half-lives passed (n) can be found by:
m = mi/2ⁿ
The mass of Pb-206 will be the mass that was lost by U-238, so it will be mi - m. Thus, the mass ration can be expressed as:
(mi-m)/m = 0.337/1
mi - m = 0.337m
mi = 1.337m
Substituing mi in the expression of half-life:
m = 1.337m/2ⁿ
2ⁿ = 1.337m/m
2ⁿ = 1.337
ln(2ⁿ) = ln(1.337)
n*ln(2) = ln(1.337)
n = ln(1.337)/ln2
n = 0.4190
The time passed (t), or the age of the sample, is the half-life time multiplied by n:
t = 4.5x10⁹ * 0.4190
t = 1.88x10⁹ ≅ 2.1x10⁹ years
The integrated rate law expression for a first order reaction is
![ln\frac{[A_{0}]}{[A_{t}]}=kt](https://tex.z-dn.net/?f=ln%5Cfrac%7B%5BA_%7B0%7D%5D%7D%7B%5BA_%7Bt%7D%5D%7D%3Dkt)
where
[A0]=100
[At]=6.25
[6.25% of 100 = 6.25]
k = 9.60X10⁻³s⁻¹
Putting values
taking log of 100/6.25
100/6.25 = 16
ln(16) = 2.7726
Time = 2.7726 / 0.0096 = 288.81 seconds
Answer:
1. c 2. b that's what i think i learned this in middle school i dont completely remember
Explanation:
When heat is absorbed by reactants in a chemical reaction then the reaction is known as endothermic reaction.
Whereas when heat is released by reactants in a chemical reaction then the reaction is known as exothermic reaction.
1. When 
is positive then it means heat is absorbed by reactants and reaction is endothermic.
2. When 
, this means energy of products is less than the energy of reactants. Therefore, it is exothermic in nature.
3. When energy is absorbed then reaction is endothermic.
4. When 
, this means energy of products is greater than the energy of reactants. Therefore, it is endothermic in nature.
5. When [tex]\Delta H is negative then it means heat is released by reactants and reaction is exothermic.
