Avogadro's number is derived from the number of carbon-12 atoms in 12.00 grams. In a chemical reaction, 3.0 moles of hydrogen react with 4.0 moles of
oxygen to produce water. How many reacting molecules are present when the
reaction starts?

answer: 45

explanationyou have to add as well to be it could be

the relative atomic mass of an element is the mass of one atom of that element, relative to 1/12th of the mass of a  1212c atom. to convert to an absolute mass in kg, it is necessary to multiply by  1.661×10−271.661×10−27.

the relative atomic mass is roughly equal to the number of neutrons, plus the number of protons, in the nucleus of the atom.

for elements with more than one stable isotope (different types of nucleus with different numbers of neutrons but the same number of protons), the quoted mass is usually the average mass, weighted by the relative abundance of the different isotopes.