Question

1) HEPES (in the HA form, MW= 238) a common buffer in biochemistry has a pKa= 7.56. How many mls of a 0.02 M NaOH solution should be added to 5.0 g of HEPES to make 200 mL of a pH 7.20 buffer?

2) Calculate the pAg values for the titration of 25.0 ml of a 0.01 M S2- solution when a) 5.0 ml, b) 25.0 ml and c) 30.0 ml of a 0.02M Ag+ solution is added. Ksp= 8.0 * (10)^-51

2Ag⁺ + S^2-  <> Ag2S

3) would adding Cr3- to water result in acidic, basic, or neutral solution? and why?

4) Determine the pH of:

15.0 mL of glacial acetic acid solution diluted to 1.50 L with water. The density of glacial acetic acid is 1.05 g/ml. Ka= 1.8 * (10)^-5

Answer 1

1) moles of HA = (mass / molar mass) x (1/ volume in L) = (5 g/ 238 g/mol)x (1/0.2 L) = 0.105 M

[HA] = molarity x volume in Litres = 0.105 M x 0.2 L = 0.021 mol

[NaOH] = molarity x volume in Litres = 0.02 M x V L = 0.02 V mol

HA           +       NaOH     --------------> NaA + H2O

0.021        0.02 V                     0

------------------------------------------------------------------------------------

0.021 -0.02V       0                            0.02V

Hence,

[HA] = 0.021 -0.02V

[NaA] = 0.02V

pH = pKa + log [NaA] /[HA]

7.2 = 7.56 + log [( 0.02 V) /(0.021 -0.02V )]

On simplification,

V = 0.32 L

V = 320 mL

Therefore,

volume of NaOH required = 320 mL