Question

Ok this is a exerpt from my lab. I prepared all my solutions correctly, and am getting ready to use the spectrophotometre. But is is asking for precise concentrations, and i am confused as two how to use the dilution formula M1V1=M2V2 to find them. Any advice would be appreciated.

You should now have six “dilute” solutions containing known concentrations of tetraaquasalicylatoiron(III) ion in addition to the stock solution. The concentrations of these dilute solutions are approximately (assumes stock solution made with 0.32 g ASA (divided by 180.17 g/mol ASA, and 0.100 L solution prepared) 1.776 10 2 mol L 1).

Sample A (5 mL stock diluted to 50 mL) 1.78 10 3 mol L 1

SampleB(5mLAdilutedto25mL) 3.56 10 4 mol L 1

SampleC(5mLBdilutedto10mL) 1.78 10 mol L 1

Sample D (3 mL stock diluted to 50 mL) 1.07 10 3 mol L 1

Sample E (1 mL stock diluted to 25 mL) 7.12 10 4 mol L 1

SampleF(1mLEdilutedto10mL) 7.12 10 5 mol L 1

The precise concentrations of all your solutions must be calculated now, before you try to use your spectrophotometer. The Vernier LoggerPro software will prompt you to enter the [solution] immediately after making your reading, and then will plot your data pair (conc., abs.).

Answer 1

You are very correct in using the relation, M1 V1 = M2 V2 in order to calculate the precise concentrations ( ie. Molarity M ,or as mol L^-1 ) of the differently diluted solutions . The following steps should lead you to desired results.

step 1, - Calculations for the molarity (M2) of stock solution,

.......... Wt . of ASA dossollved and the solution prepared by dilution to 0.1 L = 0.32 g

...................................................................................................................or, = ( 0.32 / 180.17 ) mols

.........................................................................................................................= 1.77 x 10^-2 mols

Thus 0.1 L solution contains number of mols of ASA = 1.77 x 10^-3  mols

1.00 L............................................................... = 1.77 x 10^-2 mols

So, Molarity ( M1 ) of the stock solution is.................... = 1.77 x 10^-3 M

Now , we can calculate the molarity ( M1 ) of differently diluted / prepared solutions by substituting the values for the given values for M2 , V2 & V1 . Of course M1 is to be calculated. The following table gives the desired details -

Samples......................M2 ............V2(ml)...............V1(ml)......................M1...............................................................

......A....................1.77 x 10^-2 .........5.0......................50.0....................= ( 0.0177 x 5.0 ) / 50.0 ]............................

...............................................................................................................= 1.78 x 10^-3 mol L^-1 .....................................

Similarly,

....B....................1.78 x 10^-3 ..........5.0.......................25.0.....................= 3.56 x 10^-4 mol L^-1 ....................................

....C....................3.56 x 10^-4 ..........10.0.....................5.0........................= 1.78 x 10^-4 mol L^-1 ..................................

....D....................1.78 x 10^-2 ..........3.0.......................50.0..................... = 1.07 x 10^-3 mol L^-1 ..................................

....E....................1.78 x 10^-2 .........1.0........................25.0.....................= 7.12 x 10^-4 mol L^-1 .................................

....F.....................7.12 x 10^-4 ...... ..1.0.........................10.0....................= 7.12 x10^-5 mol L^-1 .......................................

Please note that ,

1)......... solutions at A,D,& E are prepared by dilution of stock solution with M2= 1.78 x 10^-2 mol L^-1

2 ).........solutions at B, C , & F are prepared by dilution of different solutions prepared at A,B, & E

3 ) ......It seems there is a typing error while mentioning the concentrations given ( so far as the power of 10 is concerned ) . It should be a negative power to which 10 has been raised.

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