# An alkali metal weighing 6.65 grams was completely dissolved in water. 7.5 grams

An alkali metal weighing 6.65 grams was completely dissolved in water. 7.5 grams of alkali was isolated from the resulting solution by evaporation. Identify this metal.

Let’s write the reaction equation:

2M + 2H2O = 2MOH + H2 ↑, where M is an unknown alkali metal.

Amount of substance M:

v (M) = m (M) / M (M) = 6.65 / M (M) (mol).

Amount of substance MOH:

v (MOH) = m (MOH) / M (MOH) = 7.5 / (M (M) + 17) (mol).

According to the reaction equation, from 2 mol of M, 2 mol of MOH is formed, therefore:

v (M) = v (MOH),

6.65 / M (M) = 7.5 / (M (M) + 17),

M (M) = 133 (g / mol), which corresponds to the cesium metal (Cs).

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