# Homework Help: I need some help in this question please check what i have done

1. Mar 20, 2013

### lioric

1. The problem statement, all variables and given/known data
A 1.00dm3 buffer sollution contains 0.5 mol of ethanoic acid and 1.00 mol sodium ethanoate.

CH3COOH: Ka= 1.8x10-5

Calculate pH of solution
Calculate change in pH when 1cm3 of 1M HCl is added
Calculate change in pH when 2cm3 of 1.5M KOH is added

2. Relevant equations

pH=pKa + log [A-/HA]

3. The attempt at a solution

pH=pKa + log [A-/HA]

So pH= 4.7 + log [1/0.5] (I changed Ka to pKa)

pH=5.046

Is that ok?

The next part is confusing

A- + HCl → HA + Cl-

Initial conc.
A-= 1M
HCl =(What do i write for HCl conc)
HA= 0.5

2. Mar 20, 2013

### Staff: Mentor

1st part is OK.

For the second and third simply assume reaction (A- protonation or HA neutralization) went to completion and concentrations can be calculated from a simple stoichiometry. This is not 100% accurate, but works in most cases.

3. Mar 20, 2013

### epenguin

For first part doesn't [A-]/[HA] = 1?

4. Mar 20, 2013

### Staff: Mentor

Why? Questions states explicitly 1 mole of A- and 0.5 mole of HA.

5. Mar 20, 2013

### epenguin

Ah yes I had hastily read it for totals of all forms.

6. Mar 30, 2013

### lioric

thank you very much i figured it out