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 
The isotopes of an element differ in the number or neutrons whereas the number of protons or electrons of a neutral atom stays the same. The molar mass of the element is based on the percent abundances of different isotopes and the individual molar mass of each isotope.
Answer:Four types of vaccines are currently available: Live virus vaccines use the weakened (attenuated) form of the virus. The measles, mumps, and rubella (MMR) vaccine and the varicella (chickenpox) vaccine are examples
1.94 moles
I did 35/18.02 because 18.02 is the molar mass of water
