Question

In: Chemistry

Describe in detail how you would preapre 100 mL of 0.15 M HEPES buffer at pH...

Describe in detail how you would preapre 100 mL of 0.15 M HEPES buffer at pH 7.30 at room temperature. The pKa of HEPES is 7.55. Prepare the buffer by using the free acid form, and then titrate with a 1.00 M strong acid/base until the desired pH is reached

Hints:

- Calculate the amount of HEPES free acid that you need to prepare 100 mL of 0.100 M solution using the molecular weight of 238.3 g/mol

- Use the Henderson-Hasselbach equation to figure out what ratio of base form / acid form you need to prepare the desired pH

- Calculate the actual volume of 1 M NaOH you will use to bring the buffer to the desired pH

Solutions

Expert Solution

moles of HEPES = 100 x 0.1 / 1000

= 0.01

molar mass of HEPES = 238.2 g / mol

mass = moles x molar mass

= 0.01 mol x 238.2 g / mol

= 3.38 g

mass of HEPES needed = 3.38 g

moles of total buffer = 100 x 0.15 / 1000

= 0.015

x + y = 0.015 -------------------------(1)

HEPES + NaOH ---------------------------------> HEPES salt + H2O

x y

pH = pKa + log (y / x)

7.30 = 7.55 + log (y / x)

y = 0.562 x -----------------------> (2)

from 1 and 2

x = 0.00960

y = 0.00540

moles of salt = moles of NaOH

moles of NaOH = y = 0.00960

molarity of NaOH = 1 M

molarity = moles / volume

1 = 0.0960 / volume

volume = 0.0960 L

volume - 96.0 mL

volume of NaOH = 96.0 mL

queen_honey_blossom answered 1 year ago

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