The concentration of the phosphoric acid is #1.23 × 10^"-3" color(white)(l)"mol·dm"^"-3"#.
Step 1. Write the balanced equation
#M_text(r):color(white)(mll)310.18color(white)(mmmmmmmmmmmmmmmm)98.00#
#color(white)(mmm)"Ca"_3("PO"_4)_2 + "3HNO"_3 → "6Ca(NO"_3)_2 + "2H"_3"PO"_4#
Step 2. Calculate the moles of #"Ca"_3("PO"_4)_2#
#"Moles of Ca"_3("PO"_4)_2 = 7.26 color(red)(cancel(color(black)("g Ca"_3("PO"_4)_2))) × (1 "mol Ca"_3("PO"_4)_2)/(310.18 color(red)(cancel(color(black)("g Ca"_3("PO"_4)_2)))) = "0.023 41 mol Ca"_3("PO"_4)_2#
Step 3. Calculate the moles of #"H"_3"PO"_4#
#"Moles of H"_3"PO"_4 = "0.023 41" color(red)(cancel(color(black)("mol Ca"_3("PO"_4)_2))) × ("2 mol H"_3"PO"_4)/(1 color(red)(cancel(color(black)("mol Ca"_3("PO"_4)_2)))) = "0.046 81 mol H"_3
"PO"_4#
Step 4. Calculate the molarity of the #"H"_3
"PO"_4#
#"Molarity" = "moles"/"cubic decimetres" = "0.046 81 mol"/"38.0 dm"^3 = 1.23 × 10^"-3" color(white)(l)"mol·dm"^"-3"#
