A buffer must both (i) maintain a pH within ±0.10 of the pK_a of the weak acid HA and (ii) neutralize 0.0400 mol of added NaOH. Two stock solutions are available:
• 0.20 M HA at a cost of $0.04 per mmol of HA
• 0.20 M NaA at a cost of $0.06 per mmol of A⁻
If the buffer is prepared in a total volume of exactly 500 mL and the HA : A⁻ mole ratio must stay between 0.95 : 1 and 1 : 0.95, calculate the minimum total cost of reagents needed to meet both requirements.