The Kingdom of Chemithia- Part 10

You hurriedly enter the answer and as soon as you put it in, the metal plate in the wall slides up and opens. You, King Antimony, and the guards get out, and begin running like crazy away from the building. And just in time, too. The building goes up in flames before you are a kilometre away. You would have surely died if you weren't able to get out, but you are too far away now to be affected.

"Whew," says King Antimony, "That was close. Fantastic job, Aurous. We still need to get back Selene from those filthy Alkemians, though." You ask the King about the proposal from the Alkemians. "Ah yes, the pitchblende. I can't seem to figure out why they need it so badly." Suddenly you remember the Alkemian's greatest dream: to be able to find a way to convert lead into gold. They must have found out some way to do so, realizing they need energy for a nuclear reaction.

King Antimony agrees with your statement. He also thinks that the Alkemians are threatening Chemithia in order to gain their precious uranium deposits. You go back toward the ruined building to check for any clues as to where Selene has been taken to. As you go near the room with the gas tank, you see a small pile of white salt outside.

King Antimony recognizes it immediately as a mark of Selene. He says that he gave her some special salts in order to give a clue as to her location in case she was in danger. All the salts were white, but they could be analyzed in Chemithia.

You collect some of the salt and head back to Chemithia with King Antimony. You return to your laboratory as soon as you get back and begin analyzing the salt.

1. The salt reacts with dilute $H_2SO_4$ to give a colourless gas (say $X$ ) and a colourless solution (say $Y$ )
2. You react $X$ with acidified $K_2Cr_2O_7$ solution and you get a green solution and a precipitate (say $Z$ .)
3. $Z$ is burnt in air to give a gas (say $A$ ).
4. $A$ reacts with $X$ to give $Z$ and a colourless liquid. Anhydrous $CuSO_4$ turns blue when we introduce this liquid.
5. When you add aqueous $NH_3$ to $Y$ , you got a precipitate which dissolves in excess of ammonia.

What salt did Selene drop outside? Input your answer as its molecular mass rounded to the nearest integer.

This question is part of my set Tales of Chemithia ! Give it a try; it's a story woven together by Chemistry problems!