What mass of HEPES weak acid (Mr = 238.3 g/mol) should be mixed with 7.5 g of HEPES weak base (Mr = 260.3 g/mol) to prepare 350 mL of a buffer solution with a pH of 7.40? 

HEPES weak acid (K_a = 3.2 x 10^-8)

Please give your answer to two significant figures.

 Watter massa HEPES swak suur (Mr = 238.3 g/mol) moet met 7.5 g HEPES swak basis (Mr = 260.3 g/mol) gemeng word om 350 mL van 'n bufferoplossing met 'n pH van 7.40 voor te berei?

HEPES swak suur (K_a = 3.2 x 10^-8)