An alkali metal weighing 7.8 g forms 2.24 liters of hydrogen with water, what kind of metal is it?

Given:
m (Me) = 7.8 g
V (H2) = 2.24 L

To find:
Me -?

Decision:
1) Write the reaction equation:
2Me + 2H2O => 2MeOH + H2;
2) Calculate the amount of substance H2:
n (H2) = V (H2) / Vm = 2.24 / 22.4 = 0.1 mol;
3) Determine the amount of substance Me:
n (Me) = n (H2) * 2 = 0.1 * 2 = 0.2 mol;
4) Calculate the molar mass of Me:
M (Me) = m (Me) / n (Me) = 7.8 / 0.2 = 39 g / mol;
5) Using the periodic system of D.I.Mendeleev, determine the unknown metal:
Mr (Me) = M (Me) = 39 – K – potassium.

Answer: Unknown metal – K – potassium.