A 50.0 mL sample of phosphoric acid, H3PO4, is titrated with 42.50 mL of 0.250 M NaOH. What is the molarity of the acid? H3PO4 + 3NaOH = Na3PO4 + 3HOH

##0.066"mol/l"##

##H_3PO_(4(aq))+3NaOH_((aq))rarrNa_3PO_(4(aq))+3H_2O_((l))##

No. moles ##NaOH=cxxv=0.250xx0.0425=0.01##

From the equation we can see that the no. moles of phosphoric acid must be 1/3 of this:

##:.nH_3PO_4=0.01/3=0.0033##

##c=n/v##

So the concentration of the ##H_3PO_4## is given by:

##c=0.0033/0.05=0.066"mol/l"##