What is the molarity of an h3po4 solution if 15.0 ml is completely neutralized by 38.5 ml of 0.150m naoh?

The molarity  of H3PO4   solution  if   15.0  ml  is completely    neutralized by  38.5  ml  of   0.15m naoh  is  calculated   as  follows
find  moles of  NaOH  used  =   molarity  x  volume
=  38.5  x  0.15  =    5.775  moles

write   the    reacting  equation
 3NaOH  + H3PO4  =  Na3PO4  + 3H2O

from  the  equation the  reacting   ratio  between  NaOH  to  H3Po4   which  is 3:1  the  moles  of  H3PO4    is  therefore  =  5.775/3  =  1.925  moles

molarity of  H3PO4  is therefore  =  moles  /volume
=  1.925/15  =  0.128 M

Density and pressure