acid and base chemistry lab

8/12/2019 Acid and Base Chemistry Lab

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Acid - Base Chemistry and the Effect of a Buffer

Christelle Bangsi

The Acidity of Soda: An Investigation into Acids and Bases found on the Moodle site for the

winter 2014 Chemistry 230L Course.

This lab involved working with acids, bases and buffers. Its analysis also involved being

able to understand and apply knowledge of equivalence points, and how they relate to the

number of H+ions a compound or solution has. Find below, the resulting curves of all the data

collected for the different components of the lab, and later, the analysis of what it all means.

Results:

0

2

4

6

8

10

0 5 10 15 20 25

pH

Volume (mL)

The Titration Curve of Acetic Acid

Series1


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0

2

4

6

8

10

12

14

0 10 20 30 40 50

pH

Volume (mL)

Titration of Phosphoric Acid

Series1

4.46

4.48

4.5

4.52

4.54

4.56

4.58

4.6

0 1 2 3

pH

Volume (mL)

The HCl Titration of A- = [HA]

Series1


3/7

4.55

4.6

4.65

4.7

4.75

4.8

0 0.5 1 1.5 2 2.5 3

pH

Volume (mL)

The NaOH Titration of A- = [HA]

Series1

4.76

4.78

4.8

4.82

4.84

4.86

4.88

4.9

4.92

0 0.5 1 1.5 2 2.5 3

pH

Volume (mL)

The HCl Titration of A- = 2[HA]

Series1

4.85

4.9

4.95

5

5.05

0 0.5 1 1.5 2 2.5 3

pH

Volume (mL)

The NaOH Titration of A- = 2[HA]

Series1


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0

1

2

3

4

5

6

7

0 0.5 1 1.5 2 2.5 3

pH

Volume (mL)

The HCl Titration of DI Water

Series1

0

2

4

6

8

10

12

14

0 0.5 1 1.5 2 2.5 3

pH

Volume (mL)

The NaOH Titration of DI Water

Series1


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% error = ([experimentalactual] actual) 100

Ka = 10^-pKa

Acid name

and Keq

pKa Ka:

experimental

Ka: actual % error

Acetic acid 7.79 1.62 x 10-8 1.8 x 10

-5 -1.0 x 10

-9

Phosphoric

acid, Keq1

5.62 2.40 x 10 -6 7.2 x 10-3 -100

Phosphoric

acid, Keq2

10.47 3.39 x 10-11

6.3 x 10-8

-99 x 10-15

0

1

2

3

4

0 1 2 3

pH

Volume (mL)

The HCl Titration of Soda

Series1

0

2

4

6

8

10

12

0 1 2 3

pH

Volume (mL)

The NaOH Titration of Soda

Series1


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Discussion:

In both titration curves, there ultimately was an increase in pH, and both curves do have

buffer regions. For acetic acid, the start of the graph shows a consistently slow rise in pH. At the

equivalence point (when the sodium hydroxide is in excess) and beyond that point, the curve

shows that the pH rose a little bit more. For phosphoric acid, the graph is clearly broken in two

places, both of which correspond to the 2 equivalence points of that titration. At those buffer

regions, the pH rises sharply, and over a very small area. Acetic acid (CH3COOH) is a

monoprotic acid, which explains the single equivalence point, while phosphoric acid (H3PO4),

which has three H+

ions, normally has three equivalence points.

The balanced equation for the reaction of HCl and a buffer is:

H+

(aq) + Cl-(aq) + Na

+(aq) + CH3COO

-(aq) CH3COOH

+(aq)+ Na

+(aq) + Cl

-(aq)

The balanced equation for the reaction of NaOH and a buffer is:

H2O(l)+ Na+

(aq)+ CH3COO

-(aq) CH3COOH(aq)+ Na

+(aq)

+OH

-(aq)

The results for the buffer capacity for the [A-] = [HA] buffer did equal the capacity based

upon the titration curve. For both NaOH and HCl acid, after the addition of 2.5mL of base, the

buffer capacities varied only by 0.19 and 0.10 respectively compared to the titration curves.

There was a slight difference in values for both the predicted and measure values of pH

for both buffers. The predicted pH value for acetic acid was 4.76, and the measured value (for

HCl) was 4.58 for [A-] = [HA], and 4.91 for [A

-] = 2[HA]. For NaOH, the measured value was

4.57 for [A-] = [HA], and 4.87 for [A

-] = [HA].

By definition, a buffer is a solution that can be created from weak bases and their

conjugate acids, weak acids and their conjugate bases, but definitely not from strong acids and

strong bases. Sodium hydroxide is a strong base and hydrochloric acid is a strong acid. On the

other hand, acetic acid is a weak acid, and so it would create a buffer solution.

The Henderson-Hasselbach equation is an equation used for directly calculating the pH of

a buffer system: pH = pKb + log ([base]/ [acid]).


7/7

pOH = 14 - 8.50=5.50

pKb= -log [Kb] = 4.74

Given that pOH = pKb+ log ([base]/ [acid])

Log ([base]/ [acid]) = pOH - pKb= 5.50 - 4.74 =0.76

Hence 10^0.76

= 5.75 This is the ratio of [NH4Cl]/ [NH3]

To make this buffer given a solution of 0.10 M NH3(aq) and a bottle of NH4Cl(s), first

make 1L of NH4Cl(s)and then find the number of moles of NH4Cl(s), and assuming its molarity is

also 0.10M, find the number of moles by multiplying molarity and volume, which in this case

will equal 0.10 moles. # of moles = mass/molar mass, so to get the mass of NH 4Cl(s) needed,

multiply # of moles by molar mass (53.5g) which equals 5.53g , then you multiply that by the

ratio of [NH4Cl]/ [NH3], which equals 30.76g. This is the mass of NH4Cl(s)needed to add to 1L

of NH3(aq) to create the buffer solution.

Lastly, the list of ingredients in a can of Mountain Dew does show its acidity, as it

contains citric acid, and the presence of sodium citrate (which is a weak acid) can help balance

the buffer system. Based on the above titration curves involving Mountain Dew and both HCl

and NaOH, it is fair to say that, yes, it can act as a buffer, because by the end of the 50 drops in

both graphs, the pH stopped increasing or decreasing significantly.

acid and base chemistry lab

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