February 19, 2017

Homework Help: Chemistry

Posted by a Canadian on Wednesday, February 20, 2013 at 7:38pm.

You know how when you do calculations for the enhaloy change of a reaction you start sort of like this?:

NH3(aq) + HCl(aq) → NH4Cl(aq)
25 mL 25 mL
1.0 mol/L 1.0 mol/L

n = c x v
= 1 mol/L x 0.025L
= 0.025 mol

How come you only need the moles of one of the reactants? I just always did it but didn't really know why... I just know I'll remember better for a test or something if I understand, so I'd like to know. And for all our sample problems the concentrations and volumes of both reactants were conveniently the same. So we never had to choose which reactant to get the moles for. But what if the concentrations and volumes were different? Would that ever happen? Or maybe that's too advanced or something for right now? I'm just scared something like that will pop up on the quiz tomorrow and I'm just going to have to guess which reactant to use to find moles...

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