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A Model Med Exam for CLS 231

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