Citric acid (192.13 g/mol) is a triprotic acid with the following chemical formu
ID: 863949 • Letter: C
Question
Citric acid (192.13 g/mol) is a triprotic acid with the following chemical formula: H3C6O7H5.
The acid dissociation constants for citric acid are:
Ka1 = 7.1 x 10^-4 M; Ka2 = 1.7 x 10^-5 M; Ka3 = 6.4 x 10^-6 M
Describe how you would prepare at least 100.00 mL (you may make more) of a citrate buffer with a pH of about 4.40. You have available distilled water, volumetric flasks and any sodium salts you desire. You must indicate the amount in grams of material needed (except for water, you may use volume). Your designed buffer must have a reasonable buffer capacity and no solution you make can have a concentration greater than 0.500 M.
Explanation / Answer
Ka1 = 7.1 x 10-4
we know that
pKa= -logKa
so
pKa1 = -log 7.1 x 10-4 = 3.1487
pKa2 = -log 1.7 x 10-5 = 4.76955
pKa3 = -log 6.4 x 10-6 = 5.1938
given
pH of buffer = 4.4
when choosing a buffer its pKa should close to the pH
from the above we get
pKa2 is close to pH
so the required equation is
H2C6H5O7- + H2O ----> HC6H5O72- + H3O+
according to hasselbach henderson
for a acid buffer
pH = pKa + log [ conjugate base / acid ]
4.4 = 4.76955 + log[ HC6H5072- / H2C6H507-]
[ HC6H5072- / H2C6H507-] = 0.427
so
the ratio of conc of [ HC6H5072- / H2C6H507-] = 0.427
we know that
moles = conc x volume
as the final volume is same for both of them
ratio of moles = ratio of conc
so
moles of HC6H5072- / H2C6H507- = 0.427
aslo given
conc should not be greater than 0.5
we got
[ HC6H5072- / H2C6H507-] = 0.427
so [H2C6H507-] > [ HC6H5072-]
so let [H2C6H507-] = 0.5 M
final volume = 100 ml = 0.1 L
we know that
moles = conc x volume of solution (L)
moles of [H2C6H507-] = 0.5 x 0.1 = 0.05
mass of [H2C6H507-] = moles x molar mass
= 0.05 x 191.13
mass of [H2C6H507-] = 9.5565 g
from the above
moles of HC6H5072- / H2C6H507- = 0.427
so
moles of HC6H5072- = 0.427 x 0.05 = 0.02135
mass of HC6H5072- = moles x molar mass
= 0.02135 x 190.13
mass of HC6H5072- = 4.059 g
so finally to get the required buffer
take 50 ml of water then add 4.059 g of Na2(HC6H507) and then
add 9.5565 g of NaH2C6H507 . mix
now add extra water to make the final volume to 100 ml