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.
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.
Answers
DrBob222
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.
0.287 x molarmass K2HPO4 = mass K2HPO4 for the base.
Anonymous
well its asking for the concentration not the mass.
DrBob222
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>
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>