[Fzang]

Ka(H2PO4) = 6.2e-8, molar masses: KH2PO4 = 136,09 g/mole and K2HPO4 = 174,18 g/mole

A puffer mix is made of KH2PO4 and K2HPO4. A solution of 2.00 g of mix in 100 mL of water gives a pH of 6.30.

Calculate the ratio between the molar amounts of the two compounds in the puffer mix and use the result to determine the masses of KH2PO4 and K2HPO4 in the 2.00 puffer mix.

And I'm lost. I seem to get extremely tiny amounts of mass out of my calculations, so I'm not doing anything right here. What should I do?

Edit: the ratio between the molar amounts/concentration of the two compounds is [HPO4(2-] = 1.2371 * [H2PO4-] (from the puffer equation) now how do I use this?

[rabolisk]

Now you have to set up a system of equations. You are solving for masses of the two compounds. Let's say that the mass of KH₂PO₄ is x, and that of K₂HPO₄ is y. You know that the total mass is 2.00 grams, and you have figured out (via pH and pKa) what the ratio of the moles are. The ratio of the moles is not the same as the ratio of masses, but you know molar mass of both compounds.

I could just list the equations, but I think it is a valuable skill to be able to express statements in English in math using variables and constants. The rest is just algebra.

[Fzang]

So, in the puffer equation I could put masses in instead, of HPO4 and H2PO4. That would give me the ratio between their masses.

m(HPO4) = 1.2371 * m(H2PO4)

m(HPO4) + m(H2PO4) = 2.00 g

So if I insert the first in the second I'd get

1.2371 * m(H2PO4) + m(H2PO4) = 0.894 g

And the other half would just be above subtracted from total 2.00 g

I feel like this is totally not what you asked me to do, but nonetheless it seems logical to me at least. Is this anywhere near correct?

[rabolisk]

Certainly the second equation is correct, but the first is not.

The ratio of the moles is not the same as the ratio of masses, but you know molar mass of both compounds.

[Borek]

Buffer.

[Fzang]

Buffer.

Sorry. In Danish it seems like there's no logical preference between buffer/puffer, so I just translated from habit

Certainly the second equation is correct, but the first is not.

The ratio of the moles is not the same as the ratio of masses, but you know molar mass of both compounds.

Fixing..

[Fzang]

Okay, so. The ratio between moles is

n(HPO4) = 1.2371 * n(H2PO4)

And you want 2.00 g in total

m(HPO4) + m(H2PO4) = 2.00 g

The mass can also be written as n*M

n(HPO4) * M(K2HPO4) + n(H2PO4) * M(KH2PO4) = 2.00 g

That gives an equation with two unknown values. Meanwhile, from the first equation, one of the values is known, so above can be rewritten with only one unknown.

1.2371 * n(H2PO4) * M(K2HPO4) + n(H2PO4) * M(KH2PO4) = 2.00 g

Above can be solved to find n(H2PO4), from which you find the mass m(KH2PO4)

The remaining mass: m(K2HPO4) = 2.00 g - m(KH2PO4)

Yes?

[rabolisk]

Okay, so. The ratio between moles is

n(HPO4) = 1.2371 * n(H2PO4)

And you want 2.00 g in total

m(HPO4) + m(H2PO4) = 2.00 g

The mass can also be written as n*M

n(HPO4) * M(K2HPO4) + n(H2PO4) * M(KH2PO4) = 2.00 g

That gives an equation with two unknown values. Meanwhile, from the first equation, one of the values is known, so above can be rewritten with only one unknown.

1.2371 * n(H2PO4) * M(K2HPO4) + n(H2PO4) * M(KH2PO4) = 2.00 g

Above can be solved to find n(H2PO4), from which you find the mass m(KH2PO4)

The remaining mass: m(K2HPO4) = 2.00 g - m(KH2PO4)

Yes?

Correct. Beautifully done.