(I did it like this: (36/58)(500/58)moles x 22.4 cdm/mole = 120cdm.)

Hmm, I must challenge the leading factor “(36/58)” (≈ 0.621). Presumably this is the molar mass ratio of Cl to NaCl, and, if so, the reason for using it is opaque to me.

Here’s why: 500 g of salt corresponds to 500/58 = 8.62 moles of NaCl. Since each NaCl molecule contains a single Cl atom, completely dissociating the NaCl must then yield exactly 8.62 moles of

*atomic* Cl, and the Na portion is no longer of any further interest. However, since chlorine normally exists as a diatomic molecular gas, i.e. as Cl

_{2}, each mole of

*atomic* Cl yields 0.5 moles of

*molecular* chlorine gas. Therefore, the factor in question should be 0.5, and your result would then become 96.6 dm

^{3} = 96.6 litres (22.4 litres being the volume of one mole of an ideal gas at STP, not SATP). If I use STP rather than SATP in my calculation, the temperature and pressure to use are 273.15 K and 101.325 kPa (= 1 atm) respectively, and my result is then 95.9 litres, which differs by 0.7% from yours. This difference is readily explained by the slightly different molar weights for salt that were used.

How much water does South Africa save every year due to people peeing in the shower?

A few million bladder-days’ worth, roughly the same amount consumed in bars and restaurants. Or do you want that in units of camel humps per kidney failure?

But how long to drive to the nearest star?

Are you sure you calculated correctly? I get an answer that is 1/100

^{th} of that which you give — still a very long time, though.

'Luthon64