# Concentration question

1. Oct 30, 2012

### assaftolko

What is the volume you need to take from 32% HCl solution in order to get a solution in final volume of 300 ml and with Molar concentration of 0.1 M. The solution density is 1.159 gr/ml.

I don't understand why will we have change... the mass will stay the same won't it? and the density is constant... so why would the volume change?

2. Oct 30, 2012

### Staff: Mentor

Re: Stoichiometry question again

I guess question is about solution preparation - how much concentrated solution has to be diluted to 300 mL to prepare 0.1M solution. And the density given is that of the stock solution, not of the final one.

3. Oct 31, 2012

### assaftolko

Re: Stoichiometry question again

Yes I think I understood the question wrong: They want to know how much volume do I need to remove from a 32% HCl solution in order to get a solution with final volume of 300 ml and with conentration of 0.1M. I still don't know how to do this... but shouldn't the density of the solution be constant even if I extract some of the solution from the vessel it's in?

4. Oct 31, 2012

### Staff: Mentor

Re: Stoichiometry question again

As I explained before - IMHO you are not asked about REMOVING part of the solution, but about calculating how much of the solution needs to be taken and DILUTED. You are not interested in what is left, but in what is prepared. Yes, density of what is left is not changing, but it is irrelevant to the question.

5. Oct 31, 2012

### assaftolko

Re: Stoichiometry question again

you could be right, it's hard to understand their intent from how they wrote the question (which is not in English). Assuming you are correct - can you please help me to understand what are the relevant quantaties I'm interested with in order to solve this prob?

6. Oct 31, 2012

### Staff: Mentor

Re: Stoichiometry question again

How much HCl will be present in the solution after the dilution?

How much of the stock solution contains this amount of HCl?

7. Oct 31, 2012

### assaftolko

Re: Stoichiometry question again

If the concentration is 0.1M and the volume of the final solution is 300 ml - then we get:

0.1 * 0.3 = 0.03 mol of HCl in the final solution. Since the molacular weight of HCl is about 36.5 gr/mol, the mass of 0.03 mol of HCl in the final solution is: 0.03 * 36.5 = 1.095 gr.

The stock solution (I hope I understood this defenition) contains 32% HCl, so according to the weight precentage formula, in order to get 1.095 gr of HCl we have:

32 = 1.095/m * 100 where m is the mass of the stock solution. After some algebra:

m= 219/64 gr.

How am I so far?

8. Oct 31, 2012

### Staff: Mentor

Re: Stoichiometry question again

Correct.

9. Oct 31, 2012

### assaftolko

Re: Stoichiometry question again

So in order to continue I need to divide this mass in the given solution density, so the density is relevant to this question is it not?

Basicly what we did here was to take some amount of HCl 32% solution, which had the given density, and to dilute it so the final solution had concetration of 0.1M and volume of 300 ml?

10. Oct 31, 2012

### Staff: Mentor

Re: Stoichiometry question again

Yes, yes & yes.

11. Oct 31, 2012

### assaftolko

Re: Stoichiometry question again

Thank you very much!

12. Oct 31, 2012

### Staff: Mentor

Note: thread had nothing to do with stoichiometry; subject of the first post changed.