This is a mixed buffer of two systems commonly used in biology. Citric is the acid. Na2HPO4 is the base (it's not the conjugate base of citric but it does act as a weak base nevertheless ie it accepts protons to form NaH2PO4).
Demotivator ,
even if i did this right, but i don't find it logically, because when we mix citric acid and Na2HPO4, then the sodium phosphate will act as a base and takes a proton from the citric acid. this means the reaction between citric acid and sodium phosphate will act this way;
let us call citric acid as HCA,
start mol of HCA: 0.1M X0.4425L= 0.044mol.
start mol HPO4 (2-): 0.2M X 0.5575L= 0.11mol
HCA + HPO4(2-) -> CA- + H2PO4-
start: 0.044mol + 0.11mol -> 0mol + 0mol
equilibrium: 0mol+0.066mol -> 0,044mol + 0.044mol
the total volum when we mix 557.5 mL of 0.2 M Na2HPO4 to 442.5 mL of 0.1 M citric acid is 1L
at equilibrium the concentration of citric acid (HCA): 0mol / 1L =0M.
at equilibrium the concentration of HPO4 (2-): 0.066mol/1L =0.066M
this mean when i mix the solutions together, then the buffer concentration is 0.066M+0M= 0.066M, which is different than what i calculated before (0.1556M)
do you have any explanations of this, please?
thanks alot!