The two questions are:
Why is arsenous acid denoted H3AsO3?
Why is methane's molecular formula conventionally "CH₄", while water is "H₂O" (among others)?
The second question was recently closed as a duplicate of the first. I agree that the answer to the first question does contain a viable answer to the second question. However, the answer to the second question is related, but not relevant, to the first question.
I have two specific concerns about closing the second question and marking it as a duplicate of the first.
Individuals who are confused about the notation issue in the second question (mostly individuals who are beginning their studies or who have little to no chemistry background) will not understand the connection between the the two questions, as the first question appears to be about acids and the second does not appear to be about acids (even though it really is). An individual who understands the link between the two questions, likely already knows the answer to both.
The best answer to the first question includes a valid answer to the second question, but the best answer to the second question is not relevant to the first question. A good answer to a question about rectangles will out of necessity be applicable to squares, but a good answer about squares may not be relevant to rectangles. Similarly, an answer about the notation of all compounds will be applicable to a subset of compounds, but an answer restricted to that subset will not necessarily be applicable to the whole.