# Dissociation of benzoic acid

Benzoic acid is a weak acid, so it does not completely dissociate in water. I know that the dissociation equation of benzoic acid is:

$$\ce{C6H5CO2H <=> H+ + C6H5CO2-}$$

The question I'm having trouble with is:

Would the dissociated form be more or less likely to dissolve in water?

Is this basically asking if the ions on the right are more likely to dissolve when compared to $$\ce{C6H5CO2H}$$?

If this is what the question is asking, I think yes, the dissociated form will be more likely to dissolve as they are very polar when compared to $$\ce{C6H5CO2H}$$ and they can be surrounded by polar water molecules. But I'm not sure.

Solubility of benzoic acid(mw 122.12):

• 1.7 g/L (0 °C) - 0.00139 moles/100 ml
• 2.7 g/L (18 °C)
• 3.44 g/L (25 °C)
• 5.51 g/L (40 °C)
• 21.45 g/L (75 °C)
• 56.31 g/L (100 °C) - 0.04611 moles/100 ml

Solubility of sodium benzoate (mw 144.10):

• 62.69 g/100 mL (0 °C) - 0.4350 moles/100 ml
• 62.78 g/100 mL (15 °C)
• 62.87 g/100 mL (30 °C)
• 71.11 g/100 mL (100 °C) - 0.4935 moles/100 ml

So yes, sodium benzoate is more soluble than benzoic acid, but consider calcium benzoate which is less soluble.

Calcium benzoate (mw 282.31):

• 2.32 g/100 mL (0 °C) - 0.00822 moles/100 ml
• 2.72 g/100 mL (20 °C)
• 8.7 g/100 mL (100 °C) - 0.03081 moles/100 ml

So the "real" answer is - it depends...

• Interesting that unionized methyl benzoate is soluble 2.75 g/L (foodb.ca/compounds/FDB012198). It would seem that the benzoate ion is relatively compatible with water, but even more attracted to calcium dication. Solubility of unionized organic molecules (like alcohols, solubilityofthings.com/water/alcohols) decreases markedly with more than 4 carbons. Linear alkanoic acids, liquids up to nonanoic, are more polar, but drop off in solubility at hexanoic (en.wikipedia.org/wiki/Hexanoic_acid) as melting point increases. – James Gaidis May 18 '20 at 14:10

The question i'm having trouble with is:

Would the dissociated form be more or less likely to dissolve in water?

Is this basically asking if the ions on the right are more likely to dissolve when compared to $\ce{C6H5CO2H}$?

Yes, that is the gist of that question.

It basically doesn’t only touch benzoic acid or medium-chain fatty acids which are both much better soluble than the corresponding aldehydes or alcohols. It is also leading the way to things like the structure of amino acids. Take glycine. In organic solvents, it is best described as $\ce{H2N-CH2-COOH}$ while in water it assumes the form $\ce{^{+}H3N-CH2-COO-}$.