[Deafboy]

I accidentally bought an old edition of my book and it doesn't cover Material (mass) balance expressions.

Write the material (mass) balance expressions for each of the following:

A) A solution prepared by dissolving sufficient weak acid H₂A in water to yeild 100 ml of a .500 M solution

B) A saturated aqueous solutoin of Na₂HPO₄

I don't know where to start 

[Borek]

Mass balance is just a form of mass conservation - whatever you put into the system must be still there.

If you have put 1 mole of H₂A into the solution, sum of all forms of the acid - that is undissociated H₂A, HA^- and A^2- is still 1 mole.

Try to write it in the form of equation, using either concentrations or numbers of moles.