Question

How do you calculate the mass of the weak acid NaH2PO4 and the volume of the base NaOH that are needed to prepare a 50.00 mL buffer?

Note: target pH = 7.50, target total molarity = 0.150, [HA] = 0.0509 M, and [A-] = 0.0991 M

Answer 1

NaH2PO4   molarity = [HA] = 0.0509 M

volume = 50 ml = 0.05 L

molarity = moles / volume

moles = 0.0509 x 0.05 = 2.54 x 10^-3

NaH2PO4 molar mass = 120 g /mol

mass of NaH2PO4 = 2.54 x 10^-3 x 120

  mass of NaH2PO4   = 0.3054 g

HA +                  NaOH ------------------> A-

2.54 x 10^-3         n                                  0

2.54 x 10^-3-n     0                                 n

pH = pKa2 + log [NaA] / [A]

7.50 = 7.21 + log (n / 2.54 x 10^-3-n)

n / 2.54 x 10^-3-n = 1.95

n = 4.95 x 10^-3 - 1.95 n

2.95 n =4.95 x 10^-3

n = 1.68 x 10^-3

moles of NaOH = A- = 1.68 x 10^-3

molarity of A- = 0.0991 M

volume = moles / molarity

              = 1.68 x 10^-3 / 0.0991

              = 16.93

volume of NaOH = 16.93ml