The reaction equation for the thermal decomposition of alkali metal nitrate in general form:
2MNO3 (t) = 2MNO2 + O2 ↑, where M is an unknown alkali metal.
The amount of substance MNO3:
v (MNO3) = m (MNO3) / M (MNO3) = 2.9 / (M (M) + 62) (mol).
The amount of substance MNO2:
v (MNO2) = m (MNO2) / M (MNO2) = 2.44 / (M (M) + 46) (mol).
According to the composed reaction equation, from 2 mol of MNO3, 2 mol of MNO2 is formed, therefore:
v (MNO3) = v (MNO2),
2.9 / (M (M) + 62) = 2.44 / (M (M) + 46),
M (M) = 39 (g / mol), which corresponds to the metal potassium (K).
Answer: potassium (K).
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