Question

e. How many grams of HEPES (free acid) would you need to weigh out to make...

e. How many grams of HEPES (free acid) would you need to weigh out to make 1 L of 50 mM HEPES?

f. How many mL of 1 M base (NaOH or KOH) would you need to add to bring the pH to 7.4?

g. What components (and how much of each) would you add to a beaker to make 1 L 50 mM HEPES, pH 7.4.

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Homework Answers

Answer #1

e) Molar mass of HEPES (C8H18N2O4S) = 238.3 gm/mol

Number of moles of HEPES = Molarity (M) * Volume of solution (in L)

=> 50m * 1L

=> 50 m moles

Mass of HEPES required = 50 m moles * 238.3 gm/mol = 11.94 grams

f)

pH = pKa + log[base/acid]

7.4 = 7.55 = log[base/acid[

-0.15 = log[base/acid]

[base] = 10^(-0.15) * [acid] = 35.397 m moles

Volume of NaOH or KOH required = 35.397 mL of NaOH or KOH

g) Volume of NaOH or KOH added = 35.397 * (1/1.03597) = 34.167 mL of 1M NaOH

Volume of HEPES added = 1000 - 34.167mL = 965.83 mL of 50mM solution of HEPES

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