40 g of calcium (Ca) reacts with 0.4 M hydrochloric acid (HCl). The balanced equation for the reaction is:

`Ca + 2HCl -> H_2 + CaCl_2`

From the balanced reaction 1 mole of calcium requires 2 moles of pure hydrochloric acid.

The molar mass of calcium is approximately 40 g/mole. 40 g of calcium is equivalent to one mole of calcium. The complete chemical reaction of this with HCl requires two moles of hydrochloric acid.

0.4 M hydrochloric acid has 0.4 moles of the acid per liter of the solution.

2 moles HCl * (1 liter/0.4 moles) = 5 liters of solution

It requires 5 liters of 0.4 M HCl to react completely with 40 g of calcium.