Answer:
pH = 7.37
Step-by-step explanation:
The buffer H₂PO₄⁻/HPO₄²⁻ has as pKa 7.21. To find the pH of a buffer you can use H-H equation:
pH = pKa + log₁₀ [HPO₄²⁻] / [H₂PO₄⁻]
Where [HPO₄²⁻] and [H₂PO₄⁻] are molar concentrations of each species. As volume is 1.00L, [HPO₄²⁻] and [H₂PO₄⁻] are MOLES.
Moles of 25.0g of KH₂PO₄ (Molar mass: 136.086g/mol):
25.0g KH₂PO₄ ₓ (1mol / 136.086g) = 0.1837 moles KH₂PO₄ = moles H₂PO₄⁻
Moles of 38.0g of Na₂HPO₄ (Molar mass: 141.96g/mol):
38.0g KH₂PO₄ ₓ (1mol / 141.96g) = 0.2677 moles Na₂HPO₄ = moles HPO₄²⁻
Replacing in H-H equation:
pH = pKa + log₁₀ [HPO₄²⁻] / [H₂PO₄⁻]
pH = 7.21 + log₁₀ [0.2677] / [0.1837]
pH = 7.37