# Chemical Forums

## Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: ItalianChick0188 on April 09, 2009, 12:57:10 AM

Title: Calculate concentration of solution
Post by: ItalianChick0188 on April 09, 2009, 12:57:10 AM
I did a lab in Chemistry class last week called Colorimetric Determination of Aspirin and we had to prepare our own stock solution.

This is how our stock solution was made:

We weighed about 0.05 (+/- 0.005) grams of pure acetylsalicylic acid. My sample weighed 0.0512 grams. We added 10 mL of 0.5 M sodium hydroxide and then transferred this solution to a 250 mL volumetric flask. We diluted the solution with water so our total volume of solution = 250 mL.

We had to calculate the concentration of our stock solution in g/mL.  I did that and got a concentration of 2.05 X 10^-4 g/mL.

Then; we measured 1.00 mL of the ASA stock solution into a 10.0 mL volumetric flask. We then added 1.0 mL of ferric chloride-HCl-KCl buffer solution. Then we diluted the solution with water  until we got the total volume of 10 mL.

Now I'm asked to calculate the concentration of the solution containing 1.00 mL of ASA stock solution in g/mL.

I know that my volume is 10 mL. Would I do 0.0512g/10.0 mL?
That doesn't seem right seeing that I'm not incorporating the 1.00 mL of ASA stock solution.

Title: Re: Calculate concentration of solution
Post by: ItalianChick0188 on April 09, 2009, 01:47:10 AM
okay I think I might be onto something.

2.05 X 10 ^-4 * 250 mL = C 1.00 mL * 10.00 mL?

I did this because 250 mL corresponds with the concentration I calculated for it which was 2.05 X 10 ^-4.

I get 0.0513 grams. I then do 0.0513g/10.00 mL and get 0.00513?

My excel sheet keeps saying this answer is wrong but I feel like Im doing it correctly.
Title: Re: Calculate concentration of solution
Post by: Squirmy on April 09, 2009, 02:24:47 AM
okay I think I might be onto something.

2.05 X 10 ^-4 * 250 mL = C 1.00 mL * 10.00 mL?

I did this because 250 mL corresponds with the concentration I calculated for it which was 2.05 X 10 ^-4.

I get 0.0513 grams. I then do 0.0513g/10.00 mL and get 0.00513?

You're right to use C1V1 = C2V2 as this is a dilution. However, you are doing a little too much.

If you are familiar with canceling units, what units do you get for C in your calculation?

You took 1.0 mL out of 250 mL of solution and diluted to a 10-mL solution. While there are three volumes in that sentence, there's only room for two in your formula.

So, what's volumes do V1 and V2 refer to?
Title: Re: Calculate concentration of solution
Post by: ItalianChick0188 on April 09, 2009, 02:27:47 AM
Wouldn't I refer to the 10 mL and 1.00 mL? Only because Im trying to find the concentration of 1.00 mL which was taken out of the 10 mL diluted solution?
Title: Re: Calculate concentration of solution
Post by: Squirmy on April 09, 2009, 02:35:31 AM
Yep!
Title: Re: Calculate concentration of solution
Post by: Squirmy on April 09, 2009, 02:38:46 AM
Wouldn't I refer to the 10 mL and 1.00 mL? Only because Im trying to find the concentration of 1.00 mL which was taken out of the 10 mL diluted solution?
The only thing I'd change is that the question is not asking you to find the concentration of the 1.00 mL taken out of the 10 mL diluted solution. It's asking for the concentration of the solution containing the 1.00 mL of stock solution.
Title: Re: Calculate concentration of solution
Post by: ItalianChick0188 on April 09, 2009, 10:54:23 AM
okay so how exactly would I set up the equation?

0.05125grams = C10.00 * 1.00?
Title: Re: Calculate concentration of solution
Post by: ItalianChick0188 on April 09, 2009, 11:29:47 AM
So

2.05 X 10 ^ -4 = C 1.00 X 10 mL = 0.0000205 g/mL ?

So then how do I work it out if I want to find the concentration for 2.00 mL?

2.05 X 10 ^ -5 = C2.00 X 10 mL?

That doesn't seem right since I'll just be getting the same number.
Title: Re: Calculate concentration of solution
Post by: syd on April 09, 2009, 11:37:23 AM
I did a lab in Chemistry class last week called Colorimetric Determination of Aspirin and we had to prepare our own stock solution.

This is how our stock solution was made:

We weighed about 0.05 (+/- 0.005) grams of pure acetylsalicylic acid. My sample weighed 0.0512 grams. We added 10 mL of 0.5 M sodium hydroxide and then transferred this solution to a 250 mL volumetric flask. We diluted the solution with water so our total volume of solution = 250 mL.

We had to calculate the concentration of our stock solution in g/mL.  I did that and got a concentration of 2.05 X 10^-4 g/mL.

Then; we measured 1.00 mL of the ASA stock solution into a 10.0 mL volumetric flask. We then added 1.0 mL of ferric chloride-HCl-KCl buffer solution. Then we diluted the solution with water  until we got the total volume of 10 mL.

Now I'm asked to calculate the concentration of the solution containing 1.00 mL of ASA stock solution in g/mL.

I know that my volume is 10 mL. Would I do 0.0512g/10.0 mL?
That doesn't seem right seeing that I'm not incorporating the 1.00 mL of ASA stock solution.

You wouldn't want to do 0.0512g/10.0 mL, you'd only do that if you were checking your g/mL with your initial mixture that was only diluted to 10.0 mL, instead of 250.  You're right there, all you have to do is divide your total grams of acetylsalicylic acid in the new solution, by the total volume of the new solution.  Use mL, so that your answer is in g/mL.

Title: Re: Calculate concentration of solution
Post by: syd on April 09, 2009, 11:46:33 AM
So

2.05 X 10 ^ -4 = C 1.00 X 10 mL = 0.0000205 g/mL ?

so then if i want to find the concentration for 2.00 moles

it would be 0.0000205 = C 2.00 X 10 mL?

Not sure what you're doing here.  C is your concentration right?  Your question is asking you to calculate grams/mL, not molarity.  Your 2.05 x 10^-4 is grams of pure aspirin, not moles of pure aspirin
Title: Re: Calculate concentration of solution
Post by: ItalianChick0188 on April 09, 2009, 11:46:45 AM
So I figured it out for 1.00 mL?  How would I figure it out for 2.00 mL?

2.05 X 10 ^ -5 = C2.00 X 10 mL?

That doesn't seem right since I'll be dividing by 10 again to get the same numbers.
Title: Re: Calculate concentration of solution
Post by: ItalianChick0188 on April 09, 2009, 11:49:23 AM
Yes, but the concentration needs to be in g/mL. When I did it that way, my excel sheet said my answer was correct. Thats how my teacher explained to get concentration. But she has me confused when I want to find the concentration containing 2.00 mL of stock solution.

2.05 X 10 ^ -4 is the concentration I got for my stock solution. I divided that by 10 mL to get 0.000205 g/mL .

Title: Re: Calculate concentration of solution
Post by: syd on April 09, 2009, 11:53:10 AM
When I did it that way, my excel sheet said my answer was correct?

2.05 X 10 ^ -4 is the concentration I got for my stock solution. I divided that by 10 mL to get 0.000205 g/mL .

But that way doesnt make sense when I want to find the concentration of the solution containing 2.00 mL of stock solution.

Your equation is wrong, you should be multiplying your grams/mL in the stock by the number of mL and divide that result by the total volume in the new solution.

(2.05x10^-4 g)(2.00mL) = (C)(10.0 mL)
Title: Re: Calculate concentration of solution
Post by: ItalianChick0188 on April 09, 2009, 11:54:14 AM
Ohhh okay, yeah that makes a lot more sense then the examples my teacher gave me.

I feel stupid. lol.
Title: Re: Calculate concentration of solution
Post by: syd on April 09, 2009, 11:55:43 AM
Yeah I hear that.. I changed it sorry it should be (c)(volume of new solution).  That's a little less confusing