how do I work out the molecular mass of a substance such as oxide of arsenic? If I'm trying to work out the oxide part of this problem, where does hydrogen come into it?
You have to know the mass, and how many moles. There are other methods.
Hydrogen? Depends on the problem.
To determine the molecular mass of a substance like oxide of arsenic (As2O3), you first need to know the atomic masses of its constituent elements. You can find the atomic masses of elements on the periodic table.
In the case of arsenic (As), its atomic mass is approximately 74.92 grams per mole. Oxygen (O) has an atomic mass of around 16.00 grams per mole.
To calculate the molecular mass of As2O3, you multiply the atomic mass of arsenic by 2 (since there are two atoms of arsenic) and the atomic mass of oxygen by 3 (since there are three atoms of oxygen).
Therefore, the molecular mass of As2O3 is:
(2 × Atomic mass of arsenic) + (3 × Atomic mass of oxygen) = (2 × 74.92) + (3 × 16.00) = 149.84 + 48.00 = 197.84 grams per mole.
Regarding hydrogen, its relevance depends on the specific problem or substance you are working with. If you are dealing with an oxide of arsenic, hydrogen may not directly factor into the molecular mass calculation. However, if there is an element like hydrogen present in the substance, you would need to consider its atomic mass and multiply accordingly when calculating the molecular mass.