[gotalbin]

When M₂S₃ (s) is heated in air, it is converted to MO₂ . A 4.00 g sample of M₂S₃ shows a decrease in mass of 0.227 g when heated in air. What is the average atomic mass of M.

[gotalbin]

The answer is supposed to be 183 but I have no idea what my teacher is doing? Could somebody please help me?

[Borek]

Please read forum rules. Start with the reaction equation.

[gotalbin]

What did I wrong?

It says that M2S3 combines with air. What is the chemical formula for air?

[Borek]

No such thing as a chemical formula of air. However, your sulfide can react only with oxygen.

You must show that you have tried.

[gotalbin]

So M2S3 + O2 ----> MO2 + SO ?

Ive tried this problem many times - take my word for it. Its has been bothering me for weeks and I've been looking for an explanation everywhere i can.

[Borek]

Sulfur in sulfides is bivalent, just as oxygen in most oxides - that should help you determine formula of M_xO_y. What sulfur oxides do you know?

[gotalbin]

Do you mean its SO2? And I'm not trying to find the formula of MO2...only the average molecular weight of M

[Borek]

Yes, it is SO₂. You can't do the question without knowing molar mass of the oxide, that in turn requires you to know its formula.

[gotalbin]

OK, I think I have a vague notion of what to do next. Do I write out a stoic equation with the mass of M as an unknown and solve it as an algebra equation. Am I allowed to do that? And when my chemistry teacher did the problem, he did something about "cross-multiplying" the grams and the molecular weights. I completely lost him there. Do you have any idea what he is talking about.

[Borek]

OK, I think I have a vague notion of what to do next. Do I write out a stoic equation with the mass of M as an unknown and solve it as an algebra equation. Am I allowed to do that?

Sounds about correct - you have to solve algebraic equation for molar mass. Try and show what you get.