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.
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.
well its asking for the concentration not the mass.
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 calculate the quantities of K2HPO4 and KH2PO4 you need to add to the flask."
To calculate the concentrations of K2HPO4 and KH2PO4 needed to prepare a 0.50 M phosphate buffer with pH 6.77, we can use the Henderson-Hasselbalch equation:
pH = pKa + log ([A-] / [HA])
In this case, A- represents the conjugate base (K2HPO4) and HA represents the acid (KH2PO4). The pKa value is given as 6.64. Rearranging the equation, we get:
[A-] / [HA] = 10^(pH - pKa)
Now, let's calculate this ratio using the given pH value:
[A-] / [HA] = 10^(6.77 - 6.64) = 10^0.13 = 1.35
So, the ratio of [K2HPO4] to [KH2PO4] is 1.35.
To determine the actual concentrations, we need to assign a variable. Let's say the concentration of KH2PO4 is C (M), then the concentration of K2HPO4 will be 1.35C (M).
Next, we account for the total volume of the buffer solution, which is 1.000 L. Therefore, the total moles of each compound can be calculated by multiplying the concentration by the volume:
Moles of KH2PO4 = C * 1.000
Moles of K2HPO4 = 1.35C * 1.000
Since the total concentration of the buffer solution is 0.50 M, the sum of the moles of KH2PO4 and K2HPO4 must be 0.50 moles:
C * 1.000 + 1.35C * 1.000 = 0.50
Simplifying the equation:
2.35C = 0.50
C = 0.50 / 2.35 ≈ 0.213 M
Therefore, the concentration of KH2PO4 is approximately 0.213 M. And since the ratio of [K2HPO4] to [KH2PO4] is 1.35, the concentration of K2HPO4 would be 1.35 * 0.213 ≈ 0.287 M.
So, the correct concentrations of [K2HPO4] and [KH2PO4] needed to prepare the 0.50 M phosphate buffer are approximately 0.287 M and 0.213 M, respectively.