Answer
A. Yes, they are present.
B. $1.8 x 10^{-9}$
Work Step by Step
A. H$_3$O+ (hydronium) reults from water dissociating into protons and hydroxyl ions. Each proton will bind to a water molecule to form a hydronium ion. (2H$_2$O → H$_2$O + H+ + OH– → H$_3$O+ + OH–). If the pH is neutral and there is a complete lack of acid there will be more H$_3$O+ or a base, meaning there will be more OH- ions. This would mean that the H$_3$O+ and OH- ions would be equal. Being that a pH of 7.0 is as neutral as it gets, the H+ concentration would be $10^{-7}$ M. This concentration is equal to that of the H$_3$O+.
B. Knowing the concentration of water molecules will help us discern what the ratio is between the H$_3$O+ and H$_2$O molecules. Being that the molecular weight of water is 18 g/mole and 1 liter of water is 1 kg, the concentration of water would be 55.6 M (1000 [g/l]/[18 g/mole]). So, the ratio is $1.8 x 10^{-9}$, or $10^{-7}/55.6$.
