This is the boss. No calculator. Every molar mass tonight is pinned to a number you can divide in your head — caustic soda 40, calcium carbonate 100, salt 58.5 — and every tank volume is round. Nothing here needs more than mental arithmetic; it needs the steps in the right order.

Two questions, and inside each one every answer feeds the next line. The first: weigh the sack, turn it into moles, spread those moles through the make-up tank to get a stock strength, then dilute that stock into the day tank and say how many litres to pump across. Mass to moles to molarity to draw. Write each line down as you go — one slip at the top rolls all the way to the valve.

The second question runs the reactor and then asks something the arithmetic alone cannot answer. Moles charged give the mass stoichiometry allowed; the drier says what it actually gave; the two together give a percent yield. And then the last line wants a decision: does this batch ship, or does it go to the shift chemist? Watch for the trap the whole lesson has been building — a shortfall against the sheet is normal and expected, and the works wrote a percentage down precisely so that normal losses do not stop a batch. The number to compare is the percent, and the thing to compare it against is the standing order.

Everything in the chapter rides in these two questions. 5★ under par here is the chapter crown. Clock in.