[adrian_01]

 Hello everyone...it is my first time using this site and I would like to ask for help with the folowing problem:

A 40.0 ml sample of a mixture of HCl and H3PO4 is titrated with 0.100 NaOH. The first equivalence point was reached after 88.0 ml of base, and the second equivalence point after 124.6 ml of base.
a) Find initial concetration of HCl and H3PO4 in solution.
b) What %of HCl is neutralized at first equivalence point.
c) What is the concetration of H3O+ at first equivalnece point.

          Thank you sooo much for your help and I hope I'll be able to answer in my turn other questions.

[AWK]

a) First equivalent point
HCl + NaOH = NaCl + H₂O
H₃PO₄ + NaOH = NaH₂PO₄ + H₂O
Second equivalent point
NaH₂PO₄ + NaOH = Na₂HPO₄ + H2O
c_molar
40xc_HCl=(126.4-2x38.4)x0.100
40xc_H3PO4=(38.4/3)x0.100

b) 100%
c) Calculate hydrolysis of 0.00384 mole of NaH₂PO₄ in volume (40+88) mL

[Dolphinsiu]

It seems your calculation is not correct!

(a) no. of mol of Na₂HPO₄= [(124.6 - 88)/1000] x 0.1 = 3.66 x 10^-3 mol

no. of mol of NaCl = 8.8 x 10^-3- 3.66 x 10^-3 = 5.14 x 10^-3 mol

Then initial Conc. of H₃PO₄ = 3.66 x 10^-3/0.04

Then initial Conc. of HCl =  5.14 x 10^-3/0.04

(c) Calculate hydrolysis of 0.00366 mole of NaH₂PO₄ in volume (40+88) mL

[AWK]

You are right. This small error comes from using 126,4 instead of 124,6.