When the concentration of hydrogen ions in a solution is decreased by a factor of ten, the ph of the solution a. increases by 1 b. increases by 10 c. decreases by 1 d. decreases by 10?

Knowing the concentration of the hydronium ion in moles per liter or molarity, the pH is calculated using the expression
     pH = - log [H3O+]
If the concentration [H3O+] is decreased by a factor of 10, it becomes [H3O+]/10:
     pH = - log [H3O+]/10
According to the quotient rule of the properties of logarithm, we can rewrite the above equation as
     pH = -{log [H+] - log (10)] 
           = -{log [H+] - 1}
           = -log [H+] + 1
Therefore, the answer is a. increases by 1.