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 
Answer:
14
Explanation:
Atomic mass = protons + neutrons
6 + 8 = 14
2200 mg of antibiotic
Explanation:
Given that 40 mg of antibiotic/kg of the bodyweight is given.
If patient is 55 kg then the dose of antibiotic will be
if 40/1000000 is done then we can get antibiotic in kg/kg of the weight
= 0.00004 kg of antibiotic per kg
0.00004*55 ( to know how much 55 kg person will require)
= 0.0022 kg
This 0.0022 value will be converted to mg
0.0022*10^6
= 2200 mg of antibiotic will be given to a 55kg patient.