Calculate the mass of calcium required to obtain calcium hydride weighing 42 g.

Let’s write the reaction equation:

Ca + H2 (t) = CaH2

Let’s find the amount of calcium hydride substance:

v (CaH2) = m (CaH2) / M (CaH2) = 42/42 = 1 (mol).

According to the reaction equation, 1 mol of CaH2 is formed from 1 mol of Ca, therefore:

v (Ca) = v (CaH2) = 1 (mol).

Thus, the mass of calcium required for the reaction is:

m (Ca) = v (Ca) * M (Ca) = 1 * 40 = 40 (g).

Answer: 40 g.