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Calculate the volume (in mL) of a 0.630 M KOH solution that should be added to...

Calculate the volume (in mL) of a 0.630 M KOH solution that should be added to 5.750 g of HEPES (MW=238.306 g/mol, pKa = 7.56) to give a pH of 7.96.

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Expert Solution

HEPES is a weak acid you can use the Henderson equation

PH = PKa + log[X-]/[HX]
7.96 = 7.56+ log[X-]/[HX]

log[X-]/[HX] = 0.4

[X-]/[HX] = 100.4 = 2.51

HX + KOH-----> NaX + H2O
no of moles of HEPES = 5.75/238.306 = 0.0241

HX    +   KOH-----> NaX + H2O

0.0241   0.0241    0         0       initial

-x          -x          +x     +x    change

0.0241-x 0.0241-x +x    +x   at equilibrium
[X ] = 2.51[HX]
X     = 2.51(0.0241-x)

x    = 0.060491 -2.51X

3.51X   = 0.060491
X       = 0.060491/3.51

x     = 0.01723 moles

molarity = moles/volume

0.63     = 0.01723/volume

volume   = 0.01723/0.63 = 0.0273 L = 27.3 ml >>>> answer

queen_honey_blossom answered 2 years ago
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