How long will it take for 90% of the ch3cn to convert to ch3nc?

A (t) = A (o) (½) ^t/h

A (t) / Ao = 0.5^t/h

Where A (t) = amount present, A (o) = initial amount, t = elapsed time, h = half-life

If 95% has been converted then At/Ao = 0.06 t½ = 15.0 h

log (0.06) = log (0.5) t/15

15*log (0.05) / log (0.5) = t

t = 60.9 h