Acid-base equilibrium VCAA question!

[katiesaliba]

VCAA 2013 MC Q.20

I worked out the answer using the Ka value of ethanoic acid, but what's the significance of the volumes given? Also, why does concentration matter when the ratio is 1:1?

Acid-base equilibria kills me 

[Yacoubb]

VCAA 2013 MC Q.20

I worked out the answer using the Ka value of ethanoic acid, but what's the significance of the volumes given? Also, why does concentration matter when the ratio is 1:1?

Acid-base equilibria kills me 

I'd also like to know this! Has it do with water being self-ionising, and so a greater volume of the solution means more self-ionising water molecules, hence higher % ionisation? That's just my jab at it so an explanation/clarification would be great! :-)

[Scooby]

They've given the volumes as a distractor - just ignore it and look at the concentrations

[Yacoubb]

They've given the volumes as a distractor - just ignore it and look at the concentrations

I swear I was going to say that! haha yeah hate when they do that! #mindEFF

[katiesaliba]

They've given the volumes as a distractor - just ignore it and look at the concentrations

This is a silly question, but when will volume affect the Ka...if ever? Thanks

[Yacoubb]

This is a silly question, but when will volume affect the Ka...if ever? Thanks

The value of an equilbrium constant is only affected by changes in temperature. So, volume will not affect the volume of the acidity constant.

[katiesaliba]

The value of an equilbrium constant is only affected by changes in temperature. So, volume will not affect the volume of the acidity constant.

Yes true, but when working out pH will volume matter?

[Yacoubb]

Yes true, but when working out pH will volume matter?

When working out pH, volume does matter. This is looked at for instance if we diluted a sample of 1M HCl, the pH would increase due to dilution. However, the actual value of the acidity constant itself remains unchanged by anything other than changes in temperature.

[katiesaliba]

When working out pH, volume does matter. This is looked at for instance if we diluted a sample of 1M HCl, the pH would increase due to dilution. However, the actual value of the acidity constant itself remains unchanged by anything other than changes in temperature.

Ah yes! Thank you   Why exactly does diluting increase pH?

[Yacoubb]

Ah yes! Thank you   Why exactly does diluting increase pH?

When you dilute an acid, the proportion of H+ to OH- in the solution becomes closer to 1:1. Maybe this is a really simplistic answer, but it'll do. :-)

[lzxnl]

Ah yes! Thank you   Why exactly does diluting increase pH?

Diluting an acidic solution is the same as adding a solution of 10^-7 M H+, which is lower than your current concentration of H+.

And you're right, the volumes mean nothing. Only the concentration matters. As a general rule, weak acids and bases have a higher percentage ionisation when dilute.