You need to prepare an acetate buffer of pH 6.32 from a 0.800 M acetic acid solution and a 2.46 M KOHKOH solution. If you have 925 mL of the acetic acid solution, how many milliliters of the KOHKOH do you need to add to make a bufer of pH 6.32?

The pKa of acetic acid is 4.76.

1 Answer
Aug 13, 2017

293 ml

Explanation:

Ethanoic acid is a weak acid and dissociates:

sf(CH_3COOHrightleftharpoonsCH_3COO^(-)+H^+)

For which:

sf(K_a=([CH_3COO^(-)][H^+])/([CH_3COOH]))

These are equilibrium concentrations.

sf(pK_a=4.76) from which sf(K_a=1.737xx10^(-5))

Rearranging we get:

sf([H^+]=K_axx([CH_3COOH])/([CH_3COO^-]))

sf(pH=6.32) from which sf([H^+]=4.786xx10^(-7)color(white)(x)"mol/l")

:.sf(4.786xx10^(-7)=1.737xx10^(-5)xx([CH_3COOH])/([CH_3COO^(-)])

:.sf(([CH_3COOH])/([CH_3COO^(-)])=(4.786xx10^(-7))/(1.737xx10^(-5))=0.027553)

To get the required pH we need to set the acid : salt ratio to this value.

To produce the required ethanoate ions we add sf(OH^-) ions:

sf(CH_3COOH+OH^(-)rarrCH_3COO^(-)+H_2O)

You can see from the equation that the no. moles sf(OH^-) added = the no. of moles of sf(CH_3COO^(-)) produced.

Since we know sf([OH^-]) we can say that if sf(V) is the volume of the sf(OH^-) solution then:

sf(nOH^(-)=nCH_3COO_("eqm")^(-)=2.26xxV)

Where sf(n) is the no. moles.

We know the initial moles of ethanoic acid:

sf(nCH_3COOH_("init")=0.800xx0.925=0.740)

So we can get the moles after the alkali has been added. Because the dissociation is small we will assume that these are a good approximation to the equilibrium moles:

sf(nCH_3COOH_("eqm")=(0.740-2.46V))

:.sf((nCH_3COOH_("eqm"))/(nCH_3COO_("eqm")^(-))=(0.740-2.46V)/(2.46V)=0.027553)

We can use moles and not concentrations since the total volume is common to both so cancels anyway.

:.sf(0.740-2.46V=0.027553xx2.46V)

sf(0.06778V+2.46V=0.740)

sf(V=0.740/2.5277=0.293color(white)(x)L)

sf(V=293color(white)(x)"ml")

--------------------

Check by iteration:

Volume KOH = 297 ml

sf(nOH^(-)=0.293xx2.46=0.72078)

:.sf(nCH_3COO_("eqm")^(-)=0.72078)

sf(nCH_3COOH_("init")=0.740)

:.sf(nCH_3COOH_("eqm")=0.740-0.72078=0.01922)

:. sf("ratio"" "( ["acid"] )/ (["salt"]) = 0.01922/0.72078 = 0.0266655)

sf([H^+]=1.737xx10^(-5)xx0.0266655=0.046318xx10^(-5)color(white)(x)"mol/l")

sf(pH=6.33)

So not too bad.