Question

In: Chemistry

a. The ka of acetic acid is 1.8*10^-5. What is the pka of the acetic acid?...

a.

The ka of acetic acid is 1.8*10^-5.

What is the pka of the acetic acid?

b.What should be the ph of a solution of 1.0 mL of 0.1 M acetic acid and 1.0 mL of 0.1 M sodium acetate and 48.0 mL H2O?

c.What should be the ph of a solution of 5.0 ml of 0.1 M acetic acid and 5.0ml of 0.1M sodium acetate and 40.0ml h20?

is the answer 4.74 for all parts ?? thanks

Solutions

Expert Solution

a)

pKa = -log (Ka)

        = - log (1.8 x 10-5)

        = 4.74

b)

millimoles of acetic acid = Molarity x Volume = 0.1M x 1.0 mL= 0.1 millimoles

millimoles of of sodium acetate = 0.1M x 1.0 mL = 0.1 millimoles

Total volume = (1.0 + 1.0 + 48) mL = 50 mL

[CH3COOH] = [CH3COONa]= millimoles /volume = 0.1 millimoles /50 mL = 0.002M

We know that

pH = pKa + log[salt]/[acid]

= 4.74 + log (0.002/0.002)

pH = 4.74 + log(1) = 4.74

c)

millimoles of acetic acid = Molarity x Volume = 0.1M x 5.0 mL= 0.5 millimoles

millimoles of of sodium acetate = 0.1M x 5.0 mL = 0.5 millimoles

Total volume = (5.0 + 5.0 + 40) mL = 50 mL

[CH3COOH] = [CH3COONa]= millimoles /volume = 0.5 millimoles /50 mL = 0.01M

We know that

pH = pKa + log[salt]/[acid]

= 4.74 + log (0.01/0.01)

pH = 4.74 + log(1) = 4.74

Yes all parts of the question have the same answer (4.74)


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