Question

How would you predict which compound in each of the following pairs is more soluble in water? Why? a) `CH_3OCH_3` or `CH_3OH`? b) `Na_2CO_3` or `CO_2`?

Answer 1

`a.` `"Methanol......."`

Explanation:

`b.` `"Probably sodium carbonate......."`

Now methanol can engage in hydrogen bonding with water and with itself, but dimethyl ether, while polar, does not engage in hydrogen bonding with itself but does with water, and has more limited water solubility. Methanol is a water-like solvent, and is infinitely miscible in water.

As regards the solubility of sodium carbonate versus carbon dioxide, we can only assess data. Now carbon dioxide has a solubility of approx. `1.5*g*L^-1`, under standard conditions, and as a nonpolar gas we would expect this limited solubility. Sodium carbonate has a solubility of approx. `30*g*L^-1` at `298*K`. Note that we compare apples, a gas, to oranges, an ionic solute.

Note (again!) that we use `"a posteriori"` reasoning (if that is the right term!). We try to rationalize an experimental result. We DO NOT anticipate an experimental result.