Which of the following is not true about compounds?
compounds are combinations of elements that join in specific ratios.
a specific compound always has the same mass ratio of its components.
elements can combine in any proportion to form a compound.
different mass ratios of components mean different compounds.

Elements can combine in any proportion to form a compound. -third choice

C. Elements can combine in any proportion to form a compound.

Explanation:

when [oh-] is 10-5  m, and [h3o+] is 10-9  m, the poh is 5 and ph is 9. again in both cases the sum of ph and poh is 14. however, the hydrogen ion concentration is not always going to be equal to exactly 1 x 10 raised to a negative number. for example, we skipped over the value of 2.0 x 10-7.

"hydrogen bonding is the attraction of h atoms in the water molecule (slight + charge) to o atoms in a neighboring water molecule (slight - charge); it is a weak interatomic force. but it is still strong enough to the water molecules "stick" together."