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Make up two separate solutions. One of Sodium Di-Hydrogen Orthophosphate (NaH₂PO₄) and one of Di-Sodium Hydrogen Phosphate (Na₂HPO₄). Make up each to a concentration of 0.2M.

For 0.2M NaH₂PO₄:
Check the molecular weight of the “kind” of NaH₂PO₄ you are using.
For NaH₂PO₄H₂O: Mol. Weight = 137.99.
∴ 0.2M = 27.598g/L

For 0.2M Na₂HPO₄:
Check the molecular weight of the “kind” of Na₂HPO₄ you are using.
For anhydrous Na₂HPO₄: Mol. Weight = 141.96.
∴ 0.2M = 28.392g/L

Mix each solution in the following proportions to achieve the desired pH:

Thanks to Vickey, who gave me this method. Where she got it from I have no idea.