# Buffer calculation question A level chemistry help

Helllo I have been given the following question. I have been able to work out all the concentration needed to substitute in to the Ka equation expect for the [A-]. I know that it’s mols is equal to the mols of KOH (0.0604 mols) but I do not know how to go from its mols to its concentration. In the mark scheme they have multiplied the mols by 4 to get the concentration as 0.242. But where is the 4 coming from? I only need help with the conjugate salt ion concentration.

“A buffer solution is prepared by dissolving 3.39g of potassium hydroxide in 250 cm² of 0.376 moldm-3 butanoic acid. This buffer solution has a pH of 5.07 at 25°C.

Calculate the acid dissociation constant, Ka, of butanoic acid at 25 °C. Assume that the volume of the solution remains constant at 250 cm-3 when the potassium hydroxide is dissolved”
The volume used is 250cm3. 250cm3 is 0.25 dm3. So to get from moles to conentration divide by volume (0.25). Dividing by 0.25 is equivalent to multiplying by 4.
Original post by Old man1234
The volume used is 250cm3. 250cm3 is 0.25 dm3. So to get from moles to conentration divide by volume (0.25). Dividing by 0.25 is equivalent to multiplying by 4.

Perfect thank you! SO MUCH!
Original post by anonymousemouse1
Helllo I have been given the following question. I have been able to work out all the concentration needed to substitute in to the Ka equation expect for the [A-]. I know that it’s mols is equal to the mols of KOH (0.0604 mols) but I do not know how to go from its mols to its concentration. In the mark scheme they have multiplied the mols by 4 to get the concentration as 0.242. But where is the 4 coming from? I only need help with the conjugate salt ion concentration.

“A buffer solution is prepared by dissolving 3.39g of potassium hydroxide in 250 cm² of 0.376 moldm-3 butanoic acid. This buffer solution has a pH of 5.07 at 25°C.

Calculate the acid dissociation constant, Ka, of butanoic acid at 25 °C. Assume that the volume of the solution remains constant at 250 cm-3 when the potassium hydroxide is dissolved”

Can you send me the mark scheme please