Calculate the ph of 0.375 l of a 0.18 m acetic acid-0.29 m sodium acetate buffer after the addition of 0.0070 mol of hbr. assume that the volume remains constant.

We are going to use H-H equation to calculate PH:

when:

PH = Pka + ㏒[A-]/[HA]

when Pka is the dissociation constant of the weak acid = 4.75

and we have [A-] (cocentration of conjugate base - acetate) = 0.29 M

and [ HA] (concentration of weak acid- acetic acid) = 0.18 M

when the reaction equation is : 

CH3COOH + HBr ↔ CH3COO-Br  +  H2O

So, we need to calculate the moles of CH3COOH & CH3COO-Br

after adding HBr:

moles of acetic acid CH3COOH = molarity * volume

                                                       = 0.18 M * 0.375 L = 0.0675 moles

moles of acetate CH3COO- = molarity * volume 

                                                = 0.29 M * 0.375 L = 0.11 moles

so, the concentrations after adding HBr is:

[CH3COOH] =( moles CH3COOH - moles HBr)/total volume

                     = 0.0675 - 0.007 / 0.375L = 0.161 M

[CH3COO-] = (moles CH3COO-  + moles HBr)/ total volume

                    =(0.11 mol + 0.007 mol)/0.375L = 0.312 M

by substitution in H-H equation:

∴ PH = 4.75 + ㏒(0.312/0.161)

         = 5

 

The pH of 0.375 L of a 0.18 M acetic acid-0.29 M sodium acetate buffer after the addition of 0.0070 mol of HBr : 4.9

Further explanation

A buffer solution is a solution that can maintain a good pH value due to the addition of a little acid or a little base or dilution.

The buffer solution can be acidic or basic

Acid buffer solution consists of weak acids and their salts.

valence according to the amount of salt anion

Alkaline buffer solution consists of a weak base and its salt.

valence according to the amount of salt cation

If the buffer solution is acidic then

a slight addition of acid (H⁺) will be balanced by the conjugate base

the addition of a small base (OH⁻) will be balanced by the weak acid

If the buffer solution is alkaline then

the addition of a little acid (H⁺) will be balanced weak base

the addition of a small base (OH⁻) will be balanced

Acetic acid-sodium acetate is an acid buffer solution

Reaction:

CH₃COOH + NaOH ↔ CH₃COO-Na + H₂O

mole of weak acid CH₃COOH =0.18 x 0.375=0.0675

mole of CH₃COONa =0.29 x 0.375=0.11

After addition of 0.0070 mol of HBr.

Reaction :

CH₃COONa + HBr ⇔ CH₃COOH + NaBr

0.11                 0.007        0.0675

-0.007           -0.007      +0.007

====================================

0.103                0               0.0745

assumption pKa CH₃COOH = 4,76

[salt]=[CH₃COONa] = 0.103 : 0.375 = 0.275

[acid]=[CH₃COOH] = 0.0745 : 0.375 = 0.199

We can calculate [H+] :

pH = 4.76 + 0.140 = 4.9

Learn more

pH scale

link

link

solution has the lowest ph

link

Keywords : buffer, acid, salt, pH

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