This is the boss. No calculator — every draw is pinned to mental arithmetic, and the standard state does the heavy lifting: at STP one mole fills 22.4 L, and the compressor's pressures and temperatures are whole multiples of 101 kPa and 273 K, so the combined gas law becomes a pair of small fractions you can hold in your head.

One batch of gas, one long problem, and every answer feeds the next: the volume at STP, then the delivered volume after compression, then the batch's share of the header, then the pressure that share exerts, then the mass on the weighbridge. Write each line down as you go — one early slip rides all the way to the weighbridge, and the finish will trace it back for you.

Everything from the chapter is in here somewhere. 5★ under par is the chapter crown. Clock in.