Aqueous Solutions

[KDang_1D]

In reactions including water as a reactant and an aqueous solution as a product, is it safe to assume that water will always be the reactant in excess and the other reactant limiting? Otherwise how would there be enough water for the aqueous solution to dissolve in? ex. 2Na(s) + 2 H2O(l) --> 2NaOH(aq) + H2(g)

[Kaitlyn Ang 1J]

I think that if you look at the problem experimentally, then you can assume that water is not the limiting reactant because it's normally in excess, but if you're looking at the problem hypothetically (aka the hw problems we have in our textbook), you should still check to see which reactant is limiting. You can do this by using the molar mass of each reactant to determine how many moles there are, and then use the molar ratios to determine how many moles of product each reactant would create, or you can simply look at the moles of each reactant and compare their molar ratios to see which reactant has less (and therefore is limiting).

Either method works, but yes, you should always double check and not rely on our real world knowledge to make a shortcut in doing the work to solve the problems ;)

[Diana_Diep2I]

No, we cannot assume that water will always be the excess reactant. You always have to calculate for the limiting reactant because sometimes the solid does not have to dissolve completely in water. Imagine putting a little too much salt in water...you'd still have some solution and some solid at the bottom of your cup/flask.

[Katherine Brenner 3H]

This is a great question. I think for the sake of this class we should probably be careful to always assume that water will be the excess reactant, although in the real scientific world that would make a lot of sense.