The number of neutrons in an atom can be found by subtracting the atomic number from the what?

This approximation of mass can be used to easily calculate how many neutrons an element has by simply subtracting the number of protons from the mass number. Protons and neutrons both weigh about one atomic mass unit or amu. Isotopes of the same element will have the same atomic number but different mass numbers.

Explanation:

b is the correct answer

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fe 55.85 g/mol and h2o 18.02 g/mol.