He placed the following amount on square #18
2^(18 - 1) ==2^17 ==131,072 - grains of wheat
[2^64] - 1 ==18,446,744,073,709,551,615 - total number of grains of wheat on the chess board.
18,446,744,073,709,551,615 x 1 / 7000 ==2,635,249,153,387,078.80 - pounds of grain, or their total weight.