At Standard Temperature and Pressure (STP), the ideal gas equation dictates that 1 mole of any ideal gas occupies 22.4 L. Using this, the number of moles in 1.12 L of the gas is calculated as follows:
\[
\text{Moles of gas} = \frac{\text{Volume of gas}}{\text{Molar volume at STP}} = \frac{1.12}{22.4} = 0.05 \, \text{mol}
\]
The molar mass \( M \) of the gas is determined by:
\[
M = \frac{\text{Mass of gas}}{\text{Moles of gas}} = \frac{2.5 \, \text{g}}{0.05 \, \text{mol}} = 50 \, \text{g/mol}
\]
Therefore, the molar mass of the gas is 50 g/mol, under the assumption of standard conditions.