In this example, we use the density of a gas to determine the likely identity of the gas. Our problem reads, "A sample of gas has a mass of 4.29 grams and occupies 2.00 liters at a temperature of 46 degrees Celsius and a pressure of 3.50 atmospheres. You believe the gas came from one of seven tanks. Which tank did the gas likely come from: oxygen gas, ethane gas, methane gas, hydrogen gas, chlorine gas, neon gas, or argon gas?" To identify the gas, we will calculate the molar mass of the gas. We can start with the equation for the density of the gas and rearrange it to solve for the molar mass of the gas. Here, M represents the molar mass, D is density, R is the ideal gas law constant, T is the absolute temperature, and P is pressure. We're not given the density directly in this problem, but the units for density in this equation are grams per liter, so we can substitute 4.29 grams per 2.00 liters in for the density of the gas. We're also given the temperature in degrees Celsius. We can convert it to Kelvin by adding 273. And we can substitute the pressure directly into this equation since it is already given in atmospheres. Completing the calculation, we get 16.0 grams per mole. Now we just need to determine which of the seven stated gases has a matching molar mass. However, we have to be careful to remember that oxygen gas, hydrogen gas, and chlorine gas are all diatomic elements. So when we see hydrogen gas, for example, it almost always refers to H2 instead of just H. Neon and argon are noble gases, so they are monatomic elements. Based on the matching molar mass, the only gas that matches is methane, CH4.