Michael S. answered • 13d
B.S. in Chemistry, Indiana University; Organic Chem Teaching Intern
Everything here comes from one line worth memorising:
1 atm = 760 mm Hg = 101.325 kPa (101.3 is fine for most courses)
Since all three describe the same pressure, any conversion is just multiplying by the right form of 1. Set it up so the unwanted unit cancels.
To atmospheres
340 kPa × (1 atm / 101.325 kPa) = 3.4 atm
540 mm Hg × (1 atm / 760 mm Hg) = 0.711 atm
To mm Hg
1.7 atm × (760 mm Hg / 1 atm) = 1300 mm Hg (1292, rounded to two significant figures)
28.59 kPa × (760 mm Hg / 101.325 kPa) = 214.4 mm Hg
That second one needs two steps if you prefer: kPa → atm (0.2822 atm), then atm → mm Hg. Same answer — there is no direct kPa-to-mm Hg constant worth memorising, so route through atm.
To kPa
0.8 atm × (101.325 kPa / 1 atm) = 81 kPa
810 mm Hg × (101.325 kPa / 760 mm Hg) = 108 kPa
Celsius to kelvin: K = °C + 273.15
23 °C = 296 K
−100 °C = 173 K
Kelvin to Celsius: °C = K − 273.15
373 K = 100 °C (99.85 with the full 273.15 — this is the boiling point of water, which is why the number looks familiar)
100 K = −173 °C
Two things worth knowing
1. 273 or 273.15? Many courses use 273 for simplicity. Check which your teacher expects, and be consistent — the difference is 0.15 K, which almost never matters but can shift a final digit.
2. No degree sign on kelvin. It is "296 K", not "296 °K". Kelvin is an absolute scale, so its units are simply kelvins. Small thing, but exams do mark it.
A quick way to sanity-check pressure conversions: atm values are small numbers near 1, mm Hg values are in the hundreds, and kPa values are around a hundred. If you convert 340 kPa and get 3.4, that is the right neighbourhood for atm. Getting 34 000 means you multiplied where you should have divided.
