Answer: 14,348,907 grains of sand
Step-by-step explanation
We have the geometric sequence {1, 3, 9, 27, 81, 243, ...}
The nth term is 1*3^(n-1) where n is a positive integer. The last term happens when n = 16, so there will be 1*3^(16-1) = 14,348,907 grains of sand on the 16th square.