Question

You need to prepare 1.000 L (in a volumetric flask) of 0.50 M phosphate buffer, pH 6.77. Use the Henderson Hasselbalch equation with a value of 6.64 for pK2 to calculate the quantities of K2HPO4 and KH2PO4 you need to add to the flask.

What concentrations of [K2HPO4] and [KH2PO4] will you need to make the total concentration 0.50 M?
i figured out the ratio to be 1.35 and I keep getting .213M & .287M as my answers but those aren't correct.

Answer 1

I think your values are correct; you just stopped short of answering the question. You want 0.213 x molar mass KH2PO4 = mass KH2PO4 for the acid and
0.287 x molarmass K2HPO4 = mass K2HPO4 for the base.

Answer 2

well its asking for the concentration not the mass.

Answer 3

No, you're mistaken. It is asking for the mass of KH2PO4 and mass K2HPO4 that must be added to the flask to make the 0.5 M buffer.
Here is the problem with that part in bold face type and quantities in italics.

"You need to prepare 1.000 L (in a volumetric flask) of 0.50 M phosphate buffer, pH 6.77. Use the Henderson Hasselbalch equation with a value of 6.64 for pK2 to <b>calculate</b><i> the quantities</i><b> of K2HPO4 and KH2PO4 you need to add to the flask." </b>