The Student Room Group

Chemistry A2 Exam Question

Nitrates of Group 1 metals decomose in a different way from those in Group 2. Sodium nitrate on strong heating gives a pale yellow solid, E, and a colourless gas which relights a glowing splint. 7.23g of sodium nitrate gave, in such an experiment, 5.87g of E and 1.02dm^3 of gas. This gas contains no nitrogen. Deduce the equation for the thermal decomposition of sodium nitrate. (One mole of gas occupies 24dm^3 at the temperature and pressure of this experiment.)

I already know the formula 2NaNO3 -> 2NaNO2 + O2, but the question is asking to deduce it. Anyone have any ideas?
Reply 1
TheQueen1986
Nitrates of Group 1 metals decomose in a different way from those in Group 2. Sodium nitrate on strong heating gives a pale yellow solid, E, and a colourless gas which relights a glowing splint. 7.23g of sodium nitrate gave, in such an experiment, 5.87g of E and 1.02dm^3 of gas. This gas contains no nitrogen. Deduce the equation for the thermal decomposition of sodium nitrate. (One mole of gas occupies 24dm^3 at the temperature and pressure of this experiment.)

I already know the formula 2NaNO3 -> 2NaNO2 + O2, but the question is asking to deduce it. Anyone have any ideas?


Just write that it's obvious the gas is oxygen - you have deduced it, really. A deduction is different from a derivation.

Ben
Reply 2
Oh really? Cool, thanks. :smile:
Reply 3
Wait, but hang on. Why would they give me infos such as 7.23g etc.? I've never used them to get the formula, I just knew it. Surely, you need to use those figures to get the formula? It's a 5 mark question.
Reply 4
TheQueen1986
Wait, but hang on. Why would they give me infos such as 7.23g etc.? I've never used them to get the formula, I just knew it. Surely, you need to use those figures to get the formula? It's a 5 mark question.

OK - they'd like you to calculate the molar ratios of all the reacting species, so you can deduce the stoichiometry of the reaction.

Ben