The Reaction:
The reaction between calcium carbide (CaC2) and water (H2O) produces acetylene gas (C2H2) and calcium hydroxide (Ca(OH)2). The balanced chemical equation for this reaction is:

CaC2 + 2H2O → C2H2 + Ca(OH)2

Calculating the Number of Moles:
To determine the number of molecules of acetylene gas produced, we need to first calculate the number of moles of calcium carbide. Given that the molar mass of CaC2 is 64 g/mol, we can use the formula:

Number of moles = Given mass / Molar mass

Number of moles of CaC2 = 100 g / 64 g/mol = 1.5625 mol

Stoichiometric Ratio:
According to the balanced equation, 1 mole of CaC2 reacts to produce 1 mole of C2H2. Therefore, the number of moles of acetylene gas formed will also be 1.5625 mol.

Avogadro's Number:
Avogadro's number (6.022 x 10^23) represents the number of particles (atoms, molecules, etc.) present in 1 mole of a substance.

Calculating the Number of Molecules:
To calculate the number of molecules of acetylene gas produced, we can use the formula:

Number of molecules = Number of moles x Avogadro's number

Number of molecules of C2H2 = 1.5625 mol x 6.022 x 10^23 molecules/mol

Final Calculation:
Evaluating the above expression:

Number of molecules of C2H2 = 9.4 x 10^23 molecules

Therefore, in the reaction of 100g of calcium carbide with an excess of water, approximately 9.4 x 10^23 molecules of acetylene gas (C2H2) would be produced.