Your supervisor tells you to make 1.00 liter of a 50.0 mM (formal concentration)

Question

Your supervisor tells you to make 1.00 liter of a 50.0 mM (formal concentration) buffer solution, pH = 9.55. No other instructions are provided. At the end, you identify the following items:

1. Bottle of 1.0 M HCl

2. Bottle of 1.0 M NaOH

3. Bottle of sodium acetate (NaC2H3O2, FW = 82.03 g/mol)

4. Bottle of ammonium nitrate (NH4NO3, FW = 80.043 g/mol)

5. Bottle of monosodium phosphate (NaH2PO4, FW = 119.98 g/mol)

6. 100-mL graduated cylinder

7. 1-L flask with graduations

8. Box of disposable Pasteur pipets

9. pH meter 10. DI water

11. Analytical balance

Using any of the above items and the information provided, quantitatively describe how you would prepare the buffer solution that was requested by your supervisor. In other words, your answer must include the “expected” volume of HCl or NaOH, the mass of the salt you chose, and the exact procedure for making the buffer (e.g. what equipment would you use and how). Neglect activity.

Explanation / Answer

Total concentration = 50*10^-3 M

Use acid base buffer... choose NH4NO3 salt for pKb = 4.75 so pKa = 9.25 which is near to 9.55

so..

pOH = 14-9.55 = 4.45

pKb = 4.75

pOH = pKb + log(NH4+/NH3)

substitute

4.45 = 4.75 + log(NH4/NH3)

10^(4.45-4.75) = (NH4/NH3)

0.5011 = NH4+/NH3

NH3*0.5011 = NH4+

AND

NH3 + NH4+ = 50*10^-3

solve for NH4+

substitute

NH3 + (NH3*0.5011) = 50*10^-3

NH3 = (50*10^-3)/(1.5011) = 0.03330 M

NH4 = (50*10^-3) -0.03330 = 0.0167 M

so..

NH4NO3 + HCL = NH3 + NO3- + H2O

so..

mmol of NH4NO3l - mol of fHCL = 0.0167

mmol of NH3+ formed = mol of HCl

let mol of HCl = x

mmol of NH4NO3 - x = 0.0167

mmol of NH3 = x

so

we know NH3 = 0.03330 then,

NH4NO3 original =0.0167+ 0.03330 = 0.05 mmol

So..

add

0.05 mmol of NH4NO3 = mol*MW = 0.05*80.0434 = 4.00217 mg of NH4NO3

then add

0.03330 mmol of HCl

M = mol/V

V = mmol/M = 0.03330/(1) = 0.0333 mL of HCl

then add water til V = 1 L

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