This site quotes rho=1.685*g*mL^-1 for 85% H_3PO_4(aq).
And thus "mass of phosphoric acid" = 85%xx1.685*g*mL^-1, with respect to a 1*mL volume.........
And we use these data to work out the concentration in mol*L^-1.
rho="Mass of phosphoric acid"/"Volume of solution"
And thus "mass of phosphoric acid"=rhoxx"volume of solution"
((85%xx1.685*g*mL^-1xx1*mL)/(97.99*g*mol^-1))/(1xx10^-3*L)=14.6*mol*L^-1.
Given this, we can now work out the required volumes.........
C_1V_1=C_2V_2, where C_2=6.0*mol*L^-1, V_2=100*mL, V_1=??
V_1=(C_2V_2)/C_1=(6.0*mol*L^-1xx100*mL)/(14.6*mol*L^-1)=41.1*mL.
So put approx. 50*mL of water in a flask, add the required volume of acid, and then make up to 100*mL volume. You find the EXACT concentration of your solution by titration with standard base, knowing that you have a solution of Na_2HPO_4(aq) at the end point.
