CLS 231 questions and answers

Write your comments on the followings:

1-      In complexometry, EBT should be 10-100 times less stable than the metal titrant.

So metal titrant will displace the indicator from thr metal giving good and sharp EP. 

2-      Volhard method is a type of an indirect titration.

Because AgNO3 is added in excess amount and the unreacted mmol of it will be determined by titrating it with another standard (KSCN).

3-      Primary standard should have a high formula weight.

To avoid any weight error.

 

Problems:

1-     1-  How many grams of Na₂SO₄ should be weighed out to prepare 500ml of a 0.100M solution?

     Weight (g)= M x V(l) x MWt(g/mol)

                     = 0.001 x 500/1000 x 142 (g/mol)

                     = 0.071 gm

 

 

2-     2-  Calculate the pOH of A solution with [H⁺] of 4 C 10^-7 M

            pH= -log[4 × 10^-7 ] = 6.4

            pOH= 14 – 6.4 = 7.6

3-     3- Calculate the grams of analyte per gram of ppt for the following conversion:

                          Analyte                             ppt

                               P                                Ag₃PO₄

                           Bi₂S₃                              BaSO₄   

      A) Wt of P= atomic wt P / Mwt Ag₃PO₄

      30.97 / 418.58 × 1/1 = 0.07399 g = GF

 

      B) Wt of Bi₂S₃ / BaSO₄

      Mwt Bi₂S₃ /Mwt BaSO₄ × 1/3 = 0.73429 g = GF

 

4-      A 250 mL of air collected in Riyadh contains the following gases:

a)      0.000625 mL hydrogen sulfide.

b)      0.00125 mL hydrogen cynide.

       Express the conc. of each gas in ppm.

a)      ppm (gas) = µL solute / 1L air

       Hydrogen sulfide = 0.625 /0.25

                                       = 2.5 ppm

b)      hydrogen cynide = 1.25 /0.25

                         = 5ppm