Question

if you make up a solution of 100ml of 0.1m HEPES in the basic form what will be the pH? show all work

Answer 1

There are two posibilities,

1º case: 0.1m (mol/kg solvent) you need to transform from molal to molar (mol/L solution) with the following equation:

M = 1000 x d x m / (1000 + m x MW);

M = Molarity; d = density and MW = molecular weight; but we don't have density of this solution (g solution / L solution)

2º case: assuming that there is a mistake and it's 0.1 M (mol/L), then we must look for pKa of this compound (pKa HEPES = 7.55); and we must apply Henderson Hasselbalch equation, but we must know HEPES exists like a zwitterion form:

When pH = pKa exists an equilibrium between conjugate base and acid form. But we need obtain only basic form, like the following structure:

And Henderson Hasselbalch equation would be like this:

pH = pKa + Log (Basic form / Acid form)

pH = 7.55 + Log (99/1) (100/0 is not possible because we can't divided into 0)

pH = 7.55 + 1.99

pH = 9.54 (at this pH value only exists basic form of HEPES)

Then;

0.1 mol / L x 0.1 L = 0.01 mol HEPES

MW_HEPES = 238.3012 g / mol

238.3012 g / mol x 0.01 mol = 2.3830 g HEPES

Weight 2.3830 g HEPES and disolve properly in 100 mL of water, and measure pH value, and add carefully pellets of NaOH until pH 9.54; if the pH goes too high, lower it back to a pH of 7.4 by carefully adding a little HCl, while monitoring the pH.