**Step 1.** Convert grams of #"CaC"_2# to moles of #"CaC"_2#.

#2.49 cancel("g CaC₂") × ("1 mol CaC"_2)/(64.10 cancel("g CaC₂")) = "0.038 85 mol CaC"_2#

**Step 2.** Use the molar ratio from the balanced equation to convert moles of #"CaC"_2"# to moles of #"C"_2"H"_2#.

#"CaC"_2 + "2H"_2"O" → "Ca(OH)"_2 + "C"_2"H"_2"#

#0.038 85 cancel("mol CaC₂") × ("1 mol C"_2"H"_2)/(1 cancel("mol CaC₂")) = "0.038 85 mol C"_2"H"_2#

**Step 3.** Use the Ideal Gas Law to calculate the volume of #"C"_2"H"_2# at STP.

STP is defined as a pressure of 100 kPa and a temperature of 273.15 K.

#PV = nRT#

#V = (nRT)/P = (0.038 85 cancel("mol") × 8.314 cancel("kPa")"·L·"cancel("K⁻¹mol⁻¹") × 273.15cancel("K"))/(100 cancel("kPa")) = "0.882 L" = "882 mL"#

The volume at STP is 882 mL.

**Step 4.** Use the Ideal Gas Law to calculate the volume at 25 °C and 1.01 atm.

#V = (nRT)/P = (0.038 85 cancel("mol") × 0.082 06 "L·"cancel("atm·K⁻¹mol⁻¹") × 298.15cancel("K"))/(1.01 cancel("atm")) = "0.941 L" = "941 mL"#

The volume at 25 °C and 1.01 atm is 941 mL.