Question

1. Calculate the mass (in g) of solid NaCH3CO2*3H2O needed to make 150 mL of buffer solution when 10.0 mL of 3.0 M CH3COOH are used.

2. What should the pH be after 1.0 mL of 6.0 M HCl is added to 50 mL of water?

3. What should the pH be after 1.0 mL of 6.0 M NaOH is added to 50 mL of water?

4. For each of these desired pH values, choose a weak acid-conjugate base pair from your textbook that could be used to prepare a buffer solution with that pH. Calculate the desired [A-]/[HA] ratio in each case. a) pH = 6.00 b) pH = 8.00

Answer 1

1) In the buffer solution, number of moles of CH3COONa.3H2O

Number of moles of CH3COONa.3H2O = Number of moles of CH3COOH

=> 10mL/1000 * 3

=> 0.030 moles

Molar mass of CH3COONa.3H2O = 12 + 3 + 12 + 32 + 23 + 54 = 136 gm/mol

Mass of solid = 0.030 mol * 136 gm/mol = 4.08 grams

2) Molarity of H+ in the solution = number of moles of H+/Volume of solution

=> 1 mL * 6M/(1mL + 50mL)

=> 6/51 M

=> 0.1176 M

pH = -log[H+] = -log[0.1176] = 0.929

3) Molarity of OH- in the solution = number of moles of OH-/Volume of solution

=> 1 mL * 6M/(1mL + 50mL)

=> 6/51 M

=> 0.1176 M

pOH = -log[OH-] = -log[0.1176] = 0.929

pH = 14 - pOH = 14 -0.929 = 13.070