Phosphoric acid is a triprotic acid with the following pKa values:

pka1: 2.148 pka2: 7.198 pka3: 12.375

You wish to prepare 1.000 L of a 0.0100 M phosphate buffer at pH 7.45. To do this, you choose to use mix the two salt forms involved in the second ionization, NaH2PO4 and Na2HPO4, in a 1.000 L volumetric flask and add water to the mark. What mass of each salt will you add to the mixture?

?? grams NaH2PO4
?? grams Na2HPO4

Hint: Use the Henderson-Hasselbalch equation to get the molar ratio of Na2HPO4 to NaH2PO4 required, then the fraction of each form from the ratio. The total moles needed will be 1.000 L × 0.0100 M = 0.0100 moles. Use the formula mass to calculate the mass needed. (FM NaH2PO4 = 119.98; FM Na2HPO4 = 141.96).

I am completely lost on how to solve this problem.

The correct pKa value to choose is pK2.

Use the HH equation to solve for the ratio base/acid.
One equation you need is base/acid.
The other equation you need is
base + acid = 0.01
That two equations in two unknowns; solve for acid concn and base concn and convert to grams. Post your work if you gets stuck.

Setup is correct, but substitute 7.198. That should change 1.19x10^-5 to 1.78 and do what Dr.Bob222 told you to do.

if you use the pk2 wouldnt you get a negative value for nah2po4?

After you get the ratio. From the

0.01 moles-moles of Na2HPO4= moles of NaH2PO4
=> (0.01 - Na2HPO4)/(Na2HPO4) = (ratio of pka2 from hasselbalch)
unit will be mol, convert to g

Well, it seems like you're in a bit of a pickle! Chemistry can get confusing sometimes, but don't worry, I'm here to help you out with a generous sprinkling of humor!

To solve this problem, we can start by using the Henderson-Hasselbalch equation, which is like the secret recipe for buffers. Are you ready to whip up a comedic cocktail of chemicals? Let's go!

The Henderson-Hasselbalch equation is:

pH = pKa + log [A-]/[HA]

We want to prepare a buffer at pH 7.45 using the second ionization of phosphoric acid, which is between NaH2PO4 and Na2HPO4.

We know the pKa value of the second ionization of phosphoric acid is pka2 = 7.198.

So, we can plug in the given values into the Henderson-Hasselbalch equation:

7.45 = 7.198 + log [Na2HPO4]/[NaH2PO4]

Now, we can rearrange the equation to solve for [Na2HPO4]/[NaH2PO4]:

log [Na2HPO4]/[NaH2PO4] = 7.45 - 7.198 = 0.252

To get rid of the logarithm, we can convert it back to normal form:

[Na2HPO4]/[NaH2PO4] = 10^0.252

Alrighty then! Now we know the ratio of [Na2HPO4] to [NaH2PO4].

To calculate the mass, we need to consider the molar ratio and the total moles required, which is given by 1.000 L × 0.0100 M = 0.0100 moles.

Let's assume you have x moles of NaH2PO4 and y moles of Na2HPO4.

From the ratio, we have:

x/y = 10^0.252

Now, since the total moles needed is 0.0100 moles, we have:

x + y = 0.0100

So, we have a system of two equations:

x/y = 10^0.252
x + y = 0.0100

Now, to solve this system of equations, you can use your trusty calculator or any other method you prefer! Once you find the values for x and y, you can calculate the mass of each salt using their formula masses, which are FM NaH2PO4 = 119.98 and FM Na2HPO4 = 141.96.

Voila! You'll have the mass of both NaH2PO4 and Na2HPO4 salts needed for your phosphoric acid buffer mixture! Don't be too salty about it, chemistry can be a tricky business. Just remember to keep a clownish sense of humor along the way!

To solve this problem, you need to use the Henderson-Hasselbalch equation to determine the correct ratio of Na2HPO4 to NaH2PO4 needed to achieve the desired pH. The Henderson-Hasselbalch equation is as follows:

pH = pKa + log([A-]/[HA])

In this case, the pKa value you'll use is the pKa2 value of phosphoric acid since you are concerned with the second ionization. The log term will represent the ratio of Na2HPO4 to NaH2PO4.

Start by rearranging the Henderson-Hasselbalch equation:

log([A-]/[HA]) = pH - pKa2

Now you can plug in the values:

log([Na2HPO4]/[NaH2PO4]) = 7.45 - 7.198

Next, calculate the ratio of [Na2HPO4] to [NaH2PO4] by taking the antilog of both sides of the equation:

10^(7.45 - 7.198) = [Na2HPO4]/[NaH2PO4]

Simplify the equation:

10^(0.252) = [Na2HPO4]/[NaH2PO4]

[Na2HPO4]/[NaH2PO4] = 1.751

Now you know the ratio of Na2HPO4 to NaH2PO4 needed in your buffer solution is approximately 1.751 to 1.

To determine the mass of each salt needed, you can use the molar ratio in conjunction with the total moles required. The total moles needed is calculated by multiplying the desired molarity of the buffer solution (0.0100 M) by the volume (1.000 L):

Total moles = 0.0100 M x 1.000 L = 0.0100 moles

Now you can calculate the mass of each salt. Start with NaH2PO4:

Mass NaH2PO4 = moles NaH2PO4 x formula mass NaH2PO4

Mass NaH2PO4 = 0.0100 moles x 119.98 g/mol (formula mass of NaH2PO4)

Repeat the calculation for Na2HPO4:

Mass Na2HPO4 = moles Na2HPO4 x formula mass Na2HPO4

Mass Na2HPO4 = 0.0100 moles x 141.96 g/mol (formula mass of Na2HPO4)

By substituting the values, you can calculate the masses of each salt.

pH=pka+log({A-]/[HA])

pH=7.45
pka=12.375

Solve for ratio:

10^(7.45-12.375)={A-]/[HA]=1.19x10^-5

Since you have a total of 0.01 moles,

1.19x10^-5*(0.01moles)= moles of Na2HPO4

0.01 moles-moles of Na2HPO4= moles of NaH2PO4

Use the formula weights to solve for the number for each that Dr. Bob222 gave you.

**** Not sure about that pKa value that I chose.