Homework Help: CHemistry- For Dr.Bob

Posted by Robert on Thursday, August 2, 2012 at 6:01pm.

Hey Dr.Bob, I asked two days ago a question about a titration lab I had that you answered, but I still have one more question about that lab.

Here's the lab summary:

For my chemistry of solutions class, we had a titration lab where we titrated NaOH into an HCl + H2O mix. This was a preparatory titration for the next one consisting of the hydrolysis of ethyl acetate into acetic acid with H+ ions as a catalyst (in our case the HCl).

So now after I found the number of moles of NaOH and HCl (which is the same) from that solution, I had to find the mass of HCl in the 5mL HCl solution and the mass of 5mL HCl solution, which you helped me out with, but now I'm asked to find the mass of H2O in 5mL HCl solution.

For this, will I proceed like this?

Because of the equation:

HCl + NaOH --> NaCL + H2O

we know that we have 1M of each.
after using up 15mL of NaOH to neutralize the 5 mL HCl, I know that I have: 1M * 0.015L = 0.015mol * 40g/mol = 0.6mol.

moles of NaOH = 0.6mol = moles of HCl
so will the moles of water be 1-0.6 = 0.4? From there I do * 18g/mol = 7.2 g

Thanks a lot for your continuous help Dr.Bob! It's really appreciated!

CHemistry- For Dr.Bob - DrBob222, Thursday, August 2, 2012 at 6:16pm
I'm confused for several reasons. First, I don't remember the entire problem so I don't remember the molarities. Second, in the last part of this post you have "we know that we have 1 M of each" and I don't think that is right. Third, you have in this post "0.015 mol*40 g/mol = 0.6 mol". That should be 0.6 gram. If I remember you had a density of HCl solution and the mass of that solution was mass = volume x density = 5 mL x density in g/mL = ? g. You can estimate the mass H2O by (mass soln - mass HCl) in the soln.
CHemistry- For Dr.Bob - Robert, Thursday, August 2, 2012 at 6:25pm
Yes I see my mistake now, because they first ask to find the moles of NaOH used, so would it simply be 0.015 mol?

If I remember you had a density of HCl solution and the mass of that solution was mass = volume x density = 5 mL x density in g/mL = ? g. You can estimate the mass H2O by (mass soln - mass HCl) in the soln.

The problem is that I don't have the solutions total mass.

Would it actually be from the 0.015 above * 40g/mol = 0.6g. - 0.5475g (HCl)?
CHemistry- For Dr.Bob - DrBob222, Thursday, August 2, 2012 at 6:45pm
I found your question of last Tuesday, July 31. You wanted mass HCl soln. That is 1.05 x 5.00 mL = 5.25 g
mass HCl in the 5.00 mL. That = mols HCl (which you obtained from M x L) x molar mass HCl = grams HCl. Then mass H2O = 5.25 - grams HCl = mass H2O.
CHemistry- For Dr.Bob - Robert, Thursday, August 2, 2012 at 6:48pm
Awesome, it's all clear now, thanks a lot again!

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