Question

At a particular temperature, N₂O₅ decomposes according to a first-order rate law with a half-life of 3.0 s. If the initial concentration of N₂O₅ is 1.0 × 10¹⁶ molecules/cm³, what will be the concentration in molecules/cm³ after 11.5 s?

	A.	7.0 × 1014
	B.	3.4 × 101
	C.	1.0 × 1016
	D.	2.0 × 1014
	E.

Answer 1

Answer

for first order reaction  

rate constant (K)= 0.693/half life

rate constant (K)= 0.693/3 = 0.231 s^-1

now

for first order reaction

A= A0*e^-kt

here A= final concentration = ?

A0= initial concentration =1*10^16 molecules/cm^3

k= rate constant = 0.231 s^-1

t= time = 11.5 seconds

A= 1*10^16 *e^-0.231*11.5

A= 7.0*10^14 molecules/cm^3 (option A)