Insufficient data. One must also know the room's temperature (cold air is heavier) and barometric pressure. Alos, you are using different measurement scales (metric and imperial)
Assuming a seal level pressure (30.00) {15 psi) and a temp of 70 F (21 C) ... I'm going to guess somewhere around 430 lbs (give or take).
Good guess. when I posed this question and activity to my freshmen students, most of them guessed quite low and were surprised at the answer. One needs to know grams/mol of air. They had to measure the room using meter sticks. I gave them the conversions they needed and helped them with their calculations.
Here's one that might surprise many although it cannot possibly be numerical:
How heavy is a fairly dense but otherwise medium-sized cumulus cloud?
Clearly we can't quote any but very rough volumes and do not know the density of a particular cloud by droplets/volume, but on average, thousands of tonnes.
A more sensible and indeed arithmetical, question, is the weight of so-many mm of rain on a given area of land.
Interesting cloud question. I think up stuff like that out of boredom. I once tried to calculate the number of electrons it would take to completely fill the known universe. Needless to say...