Stoichiometry at STP: In Ca + 2 H2O → Ca(OH)2 + H2, how many cubic centimetres of H2 at STP are released when 8 g of Ca reacts with excess water? (Atomic weight of Ca = 40)

Introduction / Context:
Gas-evolution stoichiometry is frequently used in laboratory calculations and industrial gas balance estimations. Converting from mass of reactant to moles of gas, then to volume at standard conditions (STP), is a core skill for chemical engineering and chemistry students.

Given Data / Assumptions:

• Balanced reaction: Ca + 2 H2O → Ca(OH)2 + H2.
• Molar mass of Ca = 40 g per mol.
• STP molar volume: 22,400 c.c per mol of an ideal gas.
• Water is in excess; reaction goes to completion with respect to Ca.

Concept / Approach:
Use stoichiometry to relate moles of calcium to moles of hydrogen gas. The balanced equation shows a 1:1 molar ratio between Ca and H2. Convert the given mass of Ca to moles, then convert moles of H2 to volume at STP using the standard molar volume.

Step-by-Step Solution:
Compute moles of Ca: nCa = 8 g / 40 g mol^-1 = 0.2 mol.Use stoichiometric ratio: nH2 = nCa = 0.2 mol.Convert to volume at STP: VH2 = 0.2 mol * 22,400 c.c mol^-1 = 4,480 c.c.Select the option equal to 4,480 c.c.

Verification / Alternative check:
Dimensional sanity: 8 g is one-fifth of a mole of Ca; one-fifth of 22,400 c.c yields 4,480 c.c, matching the computed value.

Why Other Options Are Wrong:
2,240 c.c corresponds to 0.1 mol of H2; 1,120 c.c corresponds to 0.05 mol; 0.4 is a dimensionless number, not a volume.

Common Pitfalls:
Using the wrong molar mass or STP molar volume; forgetting the 1:1 stoichiometric relation; mixing litres and cubic centimetres.

Final Answer:
4480