Question

A first order reaction is 20% complete in 10 min.calculate the time for 75% completion of reaction

Answer 1

Hey friend ..
your answer is 37.5 minutes
time taken for 20% complete = 10 min
then, time taken for 10% complete= 5 min
then, time taken for 5% complete = 2.5 min
so, time taken for 75% complete = (3×10)+5+2.5 = 37.5 minutes

Answer 2

HEY THERE,

k = (2.303/t)log[R]₀/[R]

as reaction is 20%completed so , remaining conc. is 80%
here, t = 10min 
         [R]₀ = 100
         [R]  = 80
⇒ k = (2.303/10)log100/80 = 0.2303 log5/4 = 0.2303(log5 - log4)
       = 0.2303(0.6989-0.6021) = 0.2303x 0.0968  =  0.0223 min⁻¹

for 75% completion , remaining conc. is 25%
t = (2.303/k)log[R]₀/[R] = (2.303/0.0223)log100/25 = 103.27 x log4

= 103.27x0.6021 = 62.18 min 

So , your answer is  62.18 minutes 
hope , it would help you 
plz mark as brainliest