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# PH and Biochemistry

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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.

https://brainmass.com/chemistry/organic-chemistry-bonding/ph-and-biochemistry-25367

#### 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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