Half-life time of a reaction is time at which reactant concentration becomes half of its initial value.
Half-life of the first order reaction is 20 min. Rate constant can be calculated as follows:
The rate expression for first order reaction is as follows:
initial number of molecules of reactant are 
, time is 100 min thus, putting the values to calculate number of reactant at time 100 min,
![0.03466 min^{-1}=\frac{2.303}{100 min}log\frac{[10^{20}]}{A_{t}}](https://tex.z-dn.net/?f=0.03466%20min%5E%7B-1%7D%3D%5Cfrac%7B2.303%7D%7B100%20min%7Dlog%5Cfrac%7B%5B10%5E%7B20%7D%5D%7D%7BA_%7Bt%7D%7D)
On rearranging,
Or,
Therefore, number of molecules unreacted will be 
i didn’t mean to put this opps
Answer:
The answer to your question is V₁ = 12.5 ml
Explanation:
Data
Volume = V₁?
[NaOH] = C₁ = 4.0 M
Volume 2 = V₂ = 100 ml
[NaOH] = C₂ = 0.5 M
Formula of dilution
V₁C₁ = V₂C₂
Solve for V₁ (original solution)
V₁ = 
Substitution
V₁ = 
Simplification
V₁ = 
Result
V₁ = 12.5 ml
Found it http://my.ccsd.net/userdocs/documents/uGKpi9UAEwFpSIgE.pdf
