It will be at half the concentration at hour 5.
If it double every hour, this means it would have to be at half the concentration the hour before the 6th hour in order to double to get the full concentration in the sixth hour.
X2 x 2 x 2 x 2 x 2 x 2
2, 4, 8, 16, 32, 64
32 is half of 64.