128.8 g/mole.
<h3>Further explanation</h3>
<u>Given:</u>
Density of a gas = 5.75 g/l at STP
<u>Question:</u>
The molecular weight of this gas (in g/mol)
<u>The Process:</u>
This is a stoichiometric calculation.We will find out the molecular weight (or molar mass) of the gas measured at STP with known density.
Remember at STP 1 mole of gas = 22.4 L or referred to as a molar volume.
From the value of the gas density, we know that 1 L of gas has a mass of 5.75 g.
- Volume of gas = 1 L
- Mass of gas = 5.75 g
- Molar volume = 22.4 L/mole
The relationship between the mole (n), mass (m), molecular weight (M), volume (V), and molar volume (Vm) is as follows:

Let us calculate the molecular weight.




Thus the molecular weight of a gas that has a density of 5.75 g/L at STP equal to 128.8 g/mole.
- - - - - - - - - -
Notes:
- STP (in chemistry) is the abbreviation for Standard Temperature and Pressure, i.e., 273 K (0° Celsius or 32° Fahrenheit) and 1 atm.
- Remember the mole number of a substance can be found via mass, volume, or number of particles.
<h3>Learn more</h3>
- At STP, what is the volume of nitrogen gas? brainly.com/question/4294791
- Conservation of mass brainly.com/question/9473007
- What is the formula for the hydrate? brainly.com/question/13012523