# Homework Help: Adding Sodium Oxalate to hard/soft/distilled water.

1. Mar 31, 2012

1. The problem statement, all variables and given/known data

Im supposed to predict what will happen if I add Na2C2O4 to hard water, soft water, or distilled water. Hard water has Mg2+ and Ca2+ ions dissolved in it from calcium bicarbonate/carbonate and magnesium bicarbonate/carbonate.

It's a theoretical lab where we predict what would happen if we did the lab.

A student added 1 mL of hard water to one test tube, 1mL of soft water to another, and 1 mL of distilled water to a third. The hard water sample contained Ca2+ and Mg2+. The soft water contained lower concentrations of these ions.

Into each test tube, the student put two drops of 0.1 mol/L sodium oxalate solution. Then the student mixed the contents.

We didn't learn about measurements in class yet, I don't know what 0.1 mol/L means, so I'm assuming the volumes are irrelevant?

3. The attempt at a solution

Hard water:

MgCO3 (aq) + Na2C2O4 (aq) + H2O(l) -> MgC2O4 (s) + Na2CO3 (aq) + H2O(l)

(am I supposed to write H2O in the chemical equation?)

A double displacement reaction would occur because MgC2O4 is a solid so a precipitate would be formed.

Soft water:

I'm not sure what to do here.

Distilled water:

Na2C2O4 (aq) + H2O (l) -> 2NaOH (aq) + H2C2O4 (aq)

Does a reaction occur here? How do I know when an acid can decompose? It seems like it could turn in to

Na2C2O4 (aq) + H2O-> 2NaOH + H2O (l) + CO2 (g) + CO (g)

I don't completely understand how to know if a reaction occurs in a double displacement reaction with aq elements as products that are bases/acids

2. Apr 1, 2012

### Staff: Mentor

0.1 mol/L is a concentration, most likely it is irrelevant.

No, you shouldn't put water in the reaction when it is just a solvent.

Reaction is correct, but you should add another, separate reaction for calcium.

Technically these should be net ionic reactions, but if you don't know what these are, don't worry.

Honestly - I am not sure what to do too. Soft water contains both Mg2+ and Ca2+ as well, just in much lower concentration. Whether they will precipitate or not depends on the concentration of ions and solubility (more precisely - solubility product), so there is no simple answer to that.

Your other reaction is completely off, this one makes partial sense - but if you have just started to learn chemistry and you know nothing about acid/base equilibrium and hydrolysis, it is probably better to say nothing happens.

3. Apr 1, 2012

Thanks for the answer. I was taught that to predict a double displacement reaction you make the equation and a reaction occurred:

-if there's a gas liquid or solid in the products
-if both products are aq and one is a weak acid or base (can break down)

So if one product was H2CO3 it would break down in to carbon dioxide and water meaning a reaction occurred.

Also how bout this

Na2C2O4 (aq) + H2O (l) -> Na2O (aq) + H2C2O4 (aq)

Would the oxalic acid break down, or would no reaction occur?

edit, meant Na2O

Last edited: Apr 1, 2012
4. Apr 1, 2012

### Staff: Mentor

No such thing as NaO(aq). First, it is Na2O if anything, second, Na2O reacts immediately with water, so it can't exist as (aq).

Rules you have learned are not without a merit, but they are very simplified, so they can be misleading in more complicated cases.