Question

Use the Henderson-Hasselbalch equation, eq. (3), to calculate the pH expected for a buffer solution prepared from this acid and its conjugate base, assuming: the mass of the weak acid is 0.2 g and the mass of the conjugate base is 0.2 g will be added to 25 mL of water. Also assume the molar mass of the weak acid is 234 and its conjugate base is the sodium salt.

Answer 1

`pH=4.56`

Step-by-step explanation:

Hello there!

In this case, given the Henderson-Hasselbach equation, it is possible for us to compute the pH by firstly computing the concentration of the acid and the conjugate base; for this purpose we assume that the volume of the total solution is 0.025 L and the molar mass of the sodium base is 234 - 1 + 23 = 256 g/mol as one H is replaced by the Na:

`n_(acid)=(0.2g)/(234g/mol)=0.000855mol\\\\n_(base)= (0.2g)/(256g/mol)=0.000781mol`

And the concentrations are:

`[acid]=0.000855mol/0.025L=0.0342M`

`[base]=0.000781mol/0.025L=0.0312M`

Then, considering that the Ka of this acid is 2.5x10⁻⁵, we obtain for the pH:

`pH=-log(2.5x10^(-5))+log((0.0312M)/(0.0342M) )\\\\pH=4.60-0.04\\\\pH=4.56`

Best regards!