got another question for DrBob please?

Question

The composition of hydrogen selenide can be determined by heating tin (Sn) in a measured volume of gas. The hydrogen selenide is decomposed, producing solid tin selenide (SnSe) 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.

Ive to write a balanced chemical equation for the reaction between hydrogen selenide and tin remembering to include the physical states of the reactants and products. And ive to account for why the volume of gas is unchanged!!!!! HELP!!!!!!

PLEASE can u help, again??

Thanks :)

H2Se(g) + Sn(s) ==>SnSe(s) + H2(g)
You know Sn is a solid. The problem states H2Se is a gas, SnSe a solid, and H2 a gas. Why is the volume of gas is unchanged? How many mols of gas do you have on the left. One from H2Se? And how many mols of gas do you have on the right? One from H2? 'nuff said.

Mols? chemistry isnt my thing grrrr!

Im on the wrong course i kno hehe!

Wot i wrote, is all the question detailed im afraid but once again uve been a great help

Many thanks

H2Se(g) + Sn(s) ==>SnSe(s) + H2(g)
The formula tells you that 1 mol H2Se (the 1 in front of the H2Se formula) is used. The 1 for H2 on the right tells you it is 1 mole. The equation reads:
1 mol of H2Se gas reacts with 1 mol Sn solid to form 1 mol SnSe solid plus 1 mol H2 gas. A mol, by th way, is a number like a dozen (12) eggs or a gross (144) of pencils. A mol happens to be 6.02 x 10^23. That's a huge number and we call it a mol because we don't want to always say six point oh two three times ten to the twenty third. It so much easier to say "mol."