# Balanced equation

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The composition of hydrogen selenide can be determined by heating tin in a measured volume of gas. The hydrogen selenide is decomposed, producing solid tin selenide and hydrogen gas. If the temperature and pressure are unchanged, the gas volume when the decomposition is over is the same as it was to begin with. I need to work out a balanced equation for the reaction between hydrogen selenide and tin, and explain why the equation accounts for the fact that the gas volume is unchanged. So far I have H2Se + Sn = SnSe3 + H2, any help gratfully recieved.

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Remember that the volume of a gas is directly related to the number of moles of the gas, and not what the actual gas is. Therefore, one mole of hydrogen gas at the same temperature and pressure of one mole of hydrogen selenide gas will take up EXACTLY the same amount of space. (Provided that both gases are acting as ideal gases).

Your reaction looks pretty close, but make sure all the moles add up. The H2Se, Sn, and H2 are correct. So I'd double check the selenium in your equation.

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It is not even important what the precise reaction between tin and H2Se is. The only thing, what matters is that the Se apparently is taken up by the tin and what remains is H2, and most important, for one molecule of H2Se you also get one molecule of H2. As long as there is sufficient tin, you would not need that reaction equation for solving this problem.

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I don't think there's enough information to answer the question.

As it happens the formula of hydrogen selenide is H2Se This decompses to give H2 and Se. The Se then reacts with the tin I think it will give SnSe- it might be SnSe2 but it doesn't matter because it takes up vrey little space whichever compound is formed.

H2Se --> H2 + Se

The H2Se and H2 have the same volume because there are the same number of moles of each.

However, imagine that you chose H2Se2, the selenium equivalent of hydrogen peroxide.

It too will decompose to give H2 and Se (which will be removed by the tin)

H2Se2 ---> H2 + 2Se

There are still the same number of moles of gas on each side of the equation so the pressure won't change.

I'm not sure how stable H2Se2 is- it might not exist (though a web search for hydrogen diselenide does give some results so I think it's real)

The point is that it (H2Se2) would give the same result as H2Se.

You cannot distinguish the 2 possible formulae by that experiment.

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Ok, thanks for all your time and help

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