Chem 121-2010 Problem Set #9

 

1. Consider the titration of a 35.0 mL sample of 0.175 M HBr with 0.200 M KOH. Determine each of the following:

a) the initial pH

b) the volume of added base required to reach the equivalence point

c) the pH at 10.0 mL of added base

d) the pH at the equivalence point

e) the pH after adding 5.0 mL of base beyond the equivalence point

 

2. Consider the titration of a 25.0 mL sample of 0.175 M CH3NH2 with 0.150 M HBr. Determine each of the following:

a) the initial pH

b) the volume of added base required to reach the equivalence point

c) the pH at 5.0 mL of added acid

d) the pH at one-half of the equivalence point

e) the pH at the equivalence point

f) the pH after adding 5.0 mL of acid beyond the equivalence point

 

3. A 0.229 g sample of an unknown monoprotic acid is titrated with 0.112 M NaOH and required 25.0 mL of NaOH to reach the equivalence point. Determine the molar mass of the acid.

 

4. A 150.0 mL buffer solution contains 2.05 g of sodium benzoate and 2.47 g of benzoic acid. Calculate the pH of this buffer.

 

5. Write equations showing how the buffer solution in question#4 neutralizes added acid(HCl) and added base(NaOH).

 

6. Calculate the pH of the solution that results from each of the following mixtures:

a) 50.0 mL of 0.15 M HCHO2 with 75.0 mL of 0.13 M NaCHO2

b) 125.0 mL of 0.10 M NH3 with 250.0 mL of 0.10 M NH4Cl

 

7. What mass of sodium benzoate should be added to 150.0 mL of a 0.15 M benzoic acid solution in order to obtain a buffer with a pH of 4.25?

 

8. For each of the following solutions, calculate the initial pH and the final pH after adding 0.010 mol of HCl.

a) 500.0 mL of pure water

b) 500.0 mL of a buffer solution that is 0.125 M in HC2H3O2 and 0.115 M NaC2H3O2

c) 500.0 mL of a buffer solution that is 0.155 M in CH3CH2NH2 and 0.145 M in CH3CH2NH3Cl

 

9. Determine whether or not mixing the two solutions indicated below will result in a buffer.

a) 100.0 mL of 0.10 M NH3; 100.0 mL of 0.15 M NH4Cl

b) 50.0 mL of 0.10 M HCl; 35.0 mL of 0.150 M NaOH

c) 50.0 mL of 0.15 M HF; 20.0 mL of 0.10 M NaOH

d) 175.0 mL of 0.10 M NH3; 150.0 mL of 0.12 M NaOH
Answer Set for Chem 121-2010 Problem Set #9:

NOTE: Values may vary slightly depending on literature data used.

 

1.a) 0.757; b) 30.6 mL; c) 1.04; d) 7; e) 9.15

 

2.a) 11.94; b) 29.2 mL; c) 11.33; d) 10.64; e) 5.87; f) 1.90

 

3. 81.8 g/mole

 

4. 4.03

 

5. Added acid: HCl(aq) + C6H5COONa(aq) C6H5COOH(aq) + NaCl(aq)

Added base: NaOH(aq) + C6H5COOH(aq) H2O(l) + C6H5COONa(aq)

 

6.a) 3.86; b) 8.94

 

7. 3.7 g

 

8.a) 7.00, 1.70; b) 4.71, 4.56; c) 10.78, 10.66

 

9.a) yes; b) no; c) no; d) no