An alkali metal weighing 7.8 g, upon reaction with water, displaces a gas with a volume of 2.24 liters. what metal is it?

Let’s designate the desired metal through X and take into account that alkali metals are monovalent.

2X + 2H2O = 2XOH + H2.

It is known that under n.u. 1 mol of gas has a volume of 22.4 liters. From here we find how many moles of hydrogen are formed during this reaction:

2.24 = 22.4 * n.

n = 0.1 mol. Now we can find the mass of the formed H2:

m = n * Mr = 0.1 * 2 = 0.2 g. By proportion, we find the mass of the metal:

7.8 g – 0.2 g

2x – 2 g.

2x = 7.8 * 2 / 0.2 = 78. Therefore, x = 39 g. The desired metal is potassium.