Final answer:
To find the molar mass of a gas with a given density at STP, multiply the density by the molar volume of a gas at STP (22.4 L/mol). Applying this to the given density of 1.428 g/L, the molar mass is calculated to be 31.9872 g/mol, which is generally rounded to 32.0 g/mol.
Step-by-step explanation:
To calculate the molar mass of a gas with a known density at standard temperature and pressure (STP), you can use the ideal gas law concept that 1 mole of any ideal gas occupies 22.4 liters at STP.
The molar mass (MM) can be determined using the formula: MM = density at STP × molar volume at STP.
For this question, we have the density of 1.428 g/L at STP, so the molar mass would be:
MM = 1.428 g/L × 22.4 L/mol
MM = 31.9872 g/mol, which can be rounded off to 32.0 g/mol when considering significant figures.
This value can then be compared to known molar masses of gases to infer the identity of the unknown gas, if required.