A sample of gas with a mass of 250 mg occupies a volume of 217cm³ at 21.5°c and 102.5kpa. what will the density of the gas be at stp

You want help with this? What is your course calling standard temperature and pressure? Assuming it is 21.5C (it varies between courses/countries/labs but that is well within the range I tend to see) and that kpa which is about 1atm then we do not have to play with ideal gas equations, if we do have to play with ideal gas equations (protip would be to say I was learning about ideal gases and this is the question rather than hoping someone infers it) then I will need to know what they are calling standard temperature and pressure for you, actually I do not as I do not give full answers to homework but you will need to know it to work it out. I can help you convert between milligrams and grams/kilograms or cm3 (centimetres are for schoolboys and all that) but other that that density is mass divided by volume and mg/cm^3 seems reasonable for units here. Unless this is just a case of you not being used to being given "redundant" data I am really not seeing the issue. If that pressure and temperature is not stp for you then time to change units pV = nRT or Pressure x volume = no of moles x gas constant x temperature You know a setup there so you can rearrange to give you the no of moles here. That would be (pV)/(RT)=n. Though do remember to convert to Pa (multiply by 1000), metres cubed, use the right gas constant for your units (8.31441 J K^-1 mol^-1), use Kelvin (check to make sure you are not required to use more accuracy but add 273 is usually what people go with, though if stp has 0C as the temperature or you get a negative answer you will hopefully realise that in a hurrry) and convert to grams (multiply by 1000). Once you have n then you can figure out the volume for stp as it is V=(nRT)/p and then it is the standard density calculation.

Ah, I'll only be able to get that PM by at least 4:30PM GMT, because I have computer science class after school...