Please help

jenoco · Probably a bot. Joined: 11 May 2006 Posts: 1

A chemist needs 500 g of a 5% aqueous HCl solution

Instructions: Using complete sentences, describe how to make this solution. Show how to determine the amount of solute and solvent required, including all mathematical formulas and computations. Results must be labeled and include appropriate units for mass. Assume that a 37% by mass HCl stock solution will be used to prepare the 500 g of a 5% aqueous HCl solution. Be sure to also include necessary equipment and safety precautions.

shepard · Probably a bot. Joined: 04 Apr 2008 Posts: 1

Did you find an answer to: A chemist needs 500 g of a 5% aqueous HCl solution? I am taking the same course and have the same question. Could you point me in the right direction?

Hix3r · Regular Joined: 24 Apr 2007 Posts: 43 Location: Hungary

We need 500 g solution. And it has to be 5% by mass. Let's write it in equations.

HCl/(HCl+H2O) = 0.05

HCl+H2O = 500

We can easily calculate the mass of HCl that is needed.

HCl = 0.05*500 = 25 g

H2O = 500-25 = 475 g

Now we have 37% by mass HCl solution, that has water in it. So we know:

HCl/(HCl+H2O) = 0.37

Okay so how much solution should we take (HCl+H2O) to have 25 g of HCl in it? Let's make HCl+H2O=x.

25/x = 0.37
25/0.37 = x = 67.57 g

So we must take 67.57 g of HCl+H2O from the 37% solution to have 25 g og pure HCl and some plus water. The plus water is 67.57 - 25 = 42.57 g.
We took 25 g of HCl for our solution, and 42.57 g of water came with it.
We Don't need 475g of water anymore, because we already added 42.57, so the real mass of distilled water should be: 475-42.57 = 432.43 g

There is the answer: To make 500 g 5% aqueous solution of HCl, from 37% HCl solution we must take 67.57 g and add 432.43 g of distilled water.
Safety things and other tools are simple, you can do that.

Please help - have hit only dead ends on this one