Hi all! This what I've done: First I pumped underpressure into a glass container, checked the manometer's reading and then weighed the container on two different scales. Then I opened the valve a bit, checked pressure and weighed the bottle etc. I did this 10 times. So what I've got is 10 pressure-mass readings. Now the report part. I first had to draw a measured points into (p,m)-coordinate grid. Then I did linear regression and derived a formula: m=(VM)/(RT)*p so m is a function of p. With the slope (VM/RT) I could determine the mass of the air inside the bottle in room temperature and in NTP. No problems here. Next, they want me to determine the density of air with the help of VM/RT and the mass of air. It would be a fairly easy calculation if I knew the volume of the container, but I don't. Also, the molar mass of humid air (humidity 40%) is unknown. I'm completely stuck on this one. A prod into the right direction would be greatly appreciated!