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PH and Biochemistry

I need some help to get me started:

Determine the weight in grams of sodium dihydrogen phosphate (M.W.138.01) and disodium hydrogen phosphate (M.W 141.98) needed to prepare 1L of a standard buffer at pH 7.00, with a total phosphate concentration of 0.100M.

Okay I know that the pKa is going to be 6.86. I am therefore going to take the difference of pH 7 and pH 6.86 to get 0.14. I then do 10^(14) to get 1.38. However, I am lost after that step. I cannot seem to figure out the concentration. Once I know that, I can find the mass of each of the molecules.

Solution Preview

We need to consider this reaction.
H2PO4 ?? (H+) + (HPO4^2-)

Actually, the 2nd Ka for H3PO4 is 6.2x10^(-8). So, the pKa will be 7.21, not 6.86.
I found this 2nd Ka ...

Solution Summary

The masses of molecules are calculated through ph acidity levels. The solution provided takes into account the weights of sodium needed to prepare 1L of a standard buffer at certain pH levels, ultimately resulting in the concentration between pH levels and thus the mass of each of the molecules.

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