The first order reaction is 20% complete in 10 minutes. What is the rate constant of the reaction?

0.0223 min^-1

For first order reaction, 

k = \(\frac{2.303}{t}\)log\(\frac{a}{a-x}\)

k = \(\frac{2.303}{10}\)log\(\frac{100}{80}\)

k = \(\frac{2.303}{10}\)log\(\frac{5}{4}\)

k = \(\frac{2.303}{10}\)(log5 - log4)

k = \(\frac{2.303}{10}\)(0.69 - 0.60)

k = \(\frac{2.303}{10}\) x 0.09

k ≈ 0.0223 min^-1