STP stands for Standard Temperature and Pressure. Typically, the temperature is 0 degrees celsius, and 1 ATM for pressure. This helps with mole calculations because you can just use the number 22.4L because at STP, for every mole of a gas, it takes up 22.4 liters of volume. So for chlorine (Cl_2), you convert the mass to moles first. there are 35.45 grams of chlorine in one mole of chlorine. However Chlorine is typically diatomic, and exists as Cl_2, so there are actually about 70.9 grams of chlorine in 1 mole of chlorine gas. 80.09 grams chlorine / 70.9 (grams/mole) = 1.13 moles of Cl_2. To convert to liters (volume), just use the number 22.4. 1.13 moles Cl2 * 22.4 liters / mole = 25.312 liters of Cl_2 is the volume For the density question, it's a bit more tricky. You know that 1 mole is 22.4 liters, so you need to determine the mass of one mole of propane, which ends up being 44.11 grams in 22.4 liters. The more frequent unit of measurement however is X grams / 1 liter, so you need to convert to this, although 44.11 grams / 22.4 liters is technically a unit of density. To do this, simply divide both the top and bottom by 22.4. 44.11 / 22.4 = 1.97 grams. 22.4 / 22.4 = 1 liter. So your final density measurement is 1.97 grams / liter.