What is the ph of a solution with [h+]=1.25 x 10^-10

The pH of a solution is calculated as pH = -log[H+], where [H+] is the concentration of H+ ions. Based on the given concentration in the problem,
pH = -log(1.25 x 10^-10 M) = 9.90
Therefore, the pH of the solution is 9.90, and this implies a strongly basic solution.

They can measure  the amount of energy gained or lost

The density of water is 1 g/cm cubed and in order for the pebble to sink its density has to be greater than the density of water and in order for the twig to float its density has to be less than the density of water