When 2.9 g of alkali metal nitrate is heated, 2.44 g of nitrite is obtained, set the metal.

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).