1. Understand the Concepts

* Molar Mass: The mass of one mole of a substance (grams per mole, g/mol).

* STP (Standard Temperature and Pressure): 0°C (273.15 K) and 1 atm pressure.

* Ideal Gas Law: PV = nRT, where:

* P = pressure (atm)

* V = volume (L)

* n = number of moles

* R = ideal gas constant (0.0821 L⋅atm/mol⋅K)

* T = temperature (K)

2. Calculate Moles (n)

* Use the ideal gas law to find the number of moles (n) of the gas:

* n = PV / RT

* n = (1 atm)(0.250 L) / (0.0821 L⋅atm/mol⋅K)(273.15 K)

* n ≈ 0.0110 mol

3. Calculate Molar Mass

* Molar mass (M) is mass (m) divided by moles (n):

* M = m / n

* M = 1.00 g / 0.0110 mol

* M ≈ 90.9 g/mol

Therefore, the molar mass of the unknown gas is approximately 90.9 g/mol.