Answer
In total in 18 Ml water there are:
6 x $10^{24}$ protons
4.8x $10^{24}$ neutrons
6 x $10^{24}$ electrons
Work Step by Step
The sample contains 6.0 x $10^{23}$ molecules of water, with chemical formula $H_{2}$O.
For every $H_{2}$O molecule there is 1 atom O and 2 atoms H. Therefore 6.0 x $10^{23}$ molecules of water will have 6.0 x $10^{23}$ atoms O and 2 x (6.0 x $10^{23}$) atoms H.
Let us find the number of protons, neutrons, and electrons in 6.0 x $10^{23}$ atoms O. It is given that isotope oxygen-16 is present in all the molecules of water in this sample. So mass number A = 16, and we see in the periodic table of elements that Z= 8 for oxygen. So each atom oxygen has 8 protons in the nucleus, and 8 electrons in the atom. The number of neutrons would be 16 - 8 = 8 ( mass number minus atomic number, as we know that mass number is the total number of neutrons and protons in nucleus).
So for 1 atom oxygen-16, there are 8 protons, 8 neutrons, and 8 electrons.
For 6.0 x $10^{23}$ atoms oxygen-16 there should be:
8 x 6.0 x $10^{23}$ = 48x $10^{23}$ protons
8 x 6.0 x $10^{23}$ = 48x $10^{23}$ neutons
8 x 6.0 x $10^{23}$ = 48x $10^{23}$ electrons
Let us find the number of protons, neutrons, and electrons in 2x 6.0 x $10^{23}$ atoms hydrogen-1.
Number of protons for 1 atom hydrogen-1 is 1,
number of neutrons for 1 atom hydrogen-1 is 0 ( 1-1=0), because mass number is 1 and atomic number is 1.
number of electrons for 1 atom hydrogen-1 is 1.
For 2x 6.0 x $10^{23}$ atoms hydrogen-1, there are:
1 x 2x 6.0 x $10^{23}$ = 12x $10^{23}$ protons
0 x 2x 6.0 x $10^{23}$ = 0 neutrons
1 x 2x 6.0 x $10^{23}$ = 12x $10^{23}$ electrons
In total in 18 Ml water, there should be:
48x $10^{23}$ + 12x $10^{23}$ = 60 x $10^{23}$ protons
48x $10^{23}$ + 0 = 48x $10^{23}$ neutrons
48x $10^{23}$ + 12x $10^{23}$= 60 x $10^{23}$ electrons
So there are:
6 x $10^{24}$ protons
4.8x $10^{24}$ neutrons
6 x $10^{24}$ electrons