# Homework Help: Chem Equilibrium Question

1. May 3, 2017

### Not a Wrench

1. The problem statement, all variables and given/known data
Excess BaCO3 is placed in a container containing 5L of pure water. Given the following info:
BaCO3 (s) ↔ Ba2+ + CO3 2- and Kc = 5.7E-6
how many moles of BaCO3 dissolve?

2. Relevant equations
Kc=(Ba2+ )(CO3 2- ) = 5.7E-6

3. The attempt at a solution
solved for x to get .002387 moles/5L which is incorrect. Dividing by 5L gives me .000477 moles. The correct answer is .012 and I am unsure what I have done wrong. It seemed simple enough. Could someone tell me what to do?

Last edited: May 3, 2017
2. May 3, 2017

### Staff: Mentor

0.002387 is not number of moles.

3. May 3, 2017

### Not a Wrench

I forgot to add my last step but I know. It's the moles per liter. If I divide by 5 it gives me .000477 moles which is also not correct.

4. May 3, 2017

### Staff: Mentor

No, it is not moles. Check your units.

Randomly juggling numbers is NEVER a good approach to solving problems.

5. May 3, 2017

### Not a Wrench

Then what is x supposed to be? I was under the impression that the number by which it BaCO3 decreases is equal to the x that each of the products increases by.

6. May 3, 2017

### Staff: Mentor

So you are solving for x not knowing what it is? That's randomly juggling numbers, I told you it doesn't work.

You have calculated something in your first post. What it was? Can you post your whole solution?

7. May 3, 2017

### Not a Wrench

Yeah, all I did was calculate 5.7E-6 = x2 and solved for x which got me to .002387. Isn't its units M/5L because the container contains 5L of water?

8. May 3, 2017

### Staff: Mentor

No, that's not how units work. Units don't contain numbers (or, if anything, we assume all numbers to be equal to 1). Besides, typically by M we denote molar concentration (which has its own units, mol/L ← an important hint here!), not number of moles.

What is x intended to mean in this equation?

9. May 3, 2017

### Not a Wrench

The Kc equation contains (Ba2+ )(CO32- ) and they both gain x per the ice chart so therefore Kc = x2

10. May 3, 2017

### Staff: Mentor

And what you mean by (Ba2+) (or (CO32-))?

11. May 3, 2017

### Not a Wrench

Those are the products in the equilibrium. But I figured it out, anyways. I was dividing by 5L of water when I should have been multiplying by 5L because I needed to eliminate that 5L in the denominator and get moles by itself!

12. May 3, 2017

### Staff: Mentor

These are CONCENTRATIONS. And concentration has units of mol/L. Per ONE liter, not per 5 liters.

When you divided by 5 L you got the result in mol/L2 units - these don't make sense, but you have never checked so you were not able to spot the mistake.

13. May 3, 2017

### Not a Wrench

I know, thank you for the hint.

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