Urgent

[Rob_Bob]

Does changing the concentration of an acid affect its pH???

Rob =)

[smadge]

The concentration of Hydrogen ions in solution IS the pH of an acid.

[Gef]

Almost, but not quite, smvdge. The pH is the negative log to base 10 of the hydrogen ion concentration in moles per litre. For every 10 fold change in hydrogen ion concentration the pH changes by one, ie pH 2 is ten times more acidic than pH 3.

[brachiopod]

That's a bit harsh, Gef. Whether logarithmic or otherwise, however you put it, pH is a function of H+ ion concentration.

So, Rob_Bob, the p stands for concentration, H = H+ ion*, and thus altering the concentration of the acid will by definition alter the pH.

(* - sorry, anyone know the ALT code for superscript '+' ?)

[Rob_Bob]

So (eg.) if I diluted an acid, would the pH change?

=)

[NetSquirrel]

Yes, it would increase.  The more water you add to an acid solution, the closer towards 7 (neutral) the pH would become.  At least, I think this is what happens.

If this is true it would seem that the pH scale could not tell you whether an unknown solution was hydrochloric acid or vinegar - the two acids could give the same pH reading if diluted appropriately.  I'm not sure now...stupid chemistry.

[jake-the-peg]

No you're right Net_squirrel - the pH just tells you how acidic something is. Imagine a vat of vinegar and a swimming pool into which half a cup of concentrated hydrochloric acid has been poured which would you rather take a swim in?

[incitatus]

have to take issue with brachiopod:  p isnt anything to do with concentration.  it is a mathematical shorthand meaning (negative log base 10 of).  Hence pKa is -log(Ka).

pH is a logarithmic scale of H+ concentration.  But beware.  H+ conc varies with the strength of the acid (a concept very different from pH) and so one molar ethanoic acid will have a much higher pH than 1 molar HCl

[brachiopod]

Don't know what I was thinking when I wrote that 'p stands for concentration', incitatus, though a night in the pub probably helped.

However, in a 1M solution of ethanoic and a 1M hydrochloric acid, there is a potential Mole of H+ ions in each?

However, the ethanoic acid dissociates to a lesser degree than HCl, resulting in a lower concentration of H+ or [H+], and hence a higher pH (ie 'is less acidic').

I take it you were introducing the concept of 'weak' and 'strong' acids by virtue of their dissociation constants?

[Rob_Bob]

Basically, I'm doing a Biology coursework project about the affect of acid rain on the rate of respiration in yeast. This rate of respiration is recorded by measuring the amount of carbon dioxide produced as a by-product of the respiration. In the experiment, we mix 2 Molar nitric acid, water, and yeast in the following ratios:

• Y : W : A     (Molarity)
• 5 : 10 : 0     (0 Molar acid)
• 5 : 8 : 2       (0.4 M)
• 5 : 6 : 4       (0.8 M)
• 5 : 4 : 6       (1.2 M)
• 5 : 2 : 8       (1.6 M)
• 5 : 0 : 10     (2.0 M)

This means that there are different concentrations of the acid mixed with the yeast, but the overall volume of the mixture is always the same. I know that the more concentrated the acid is (the higher the molarity), the slower the rate of respiration is - but for the Theory section of the coursework, I need to explain this in terms of pH and concentration.

Basically, I know the enzymes that control respiration become denatured when hydrogen ions in the acid combine with amino acid chains in the enzyme, and disrupt it. I also know that the lower the pH (the more acidic it is) the higher the concentration of hydrogen ions, and therefore more enzymes are denatured and so the rate of respiration is slower.

However, all I need to know now is: does changing the concentration of the acid (as in my experiment) affect the pH, and therefore the concentration of hydrogen ions??

Many thanks,

Rob =)

[Gef]

Yes Rob. As you increase the concentration of the acid, the concentration of hydrogen ions increases and therefore the pH decreases. In your first experiment (no acid) the pH should be 7 and in your last experiment (no water) the pH would be between 0 and -1 (sorry but haven't got a calculator handy for the exact value).