Calculate the mass of sodium acetate needed to make buffer B. Calculate the mass

Question

Calculate the mass of sodium acetate needed to make buffer B.

Calculate the mass of sodium acetate (NaC2H3O2) that must be added to make 100 mL of an acetic acid/acetate buffer at pH = 5.0, given that you will use 5.0 mL of 0.50 M acetic acid.

Calculatetheconcentrationofaceticacidafter5.0mLof0.50Maceticacidhas been diluted to 100.0 mL.

[HC2H3O2] = ____________ M

UsingtheHenderson-Hasselbalchequation,calculatetheconcentrationof NaC2H3O2 need in the buffer.

[NaC2H3O2] = ____________ M

Calculate the mass of NaC2H3O2 needed to make 100.0 mL of solution of the concentration you determined in part 2.

Mass = ________________ g

IfyouneededabufferofpH5.5,wouldyourequiremoreorfewergramsof NaC2H3O2? Why?

Explanation / Answer

pKa for Acetic Acid = 4.76

Moles of Acetic Acid added = Conc. x Vol = 0.5 x 5 x 10^-3 = 2.5 x 10^-3 moles

According to Henderson Equation,

pH = pKa + log (Salt / Acid)

=> 5 = 4.76 + log (Salt / 2.5 x 10^-3)

=> Salt = 4.345 x 10^-3 moles = Moles of Sodium Acetate

Molar Mass of CH3COONa = 82.03

Therefore, Mass of CH3COONa = 4.345 x 10^-3 x 82.03 = 0.3564 g

Conc. of Acetic Acid after dilution,

Conc. = Moles / Volume (L) = 2.5 x 10^-3 / 0.1 = 0.025 M

Conc. of NaC2H3O2 needed in the buffer,

Conc. = Moles / Volume (L) = 4.345 x 10^-3 / 0.1 = 0.04345 M

Mass of NaC2H3O2 needed = 0.3564 g (as calculated above)

If we need a buffer with pH = 5.5, we would require more grams of CH3COONa. Adding CH3COONa decreases the H+ conc. in the soultion (It acts as a weak base). As H+ conc. decreases pH increases.

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