Making Dilutions From Acid and Base Solutions

Making Dilutions From Acid and Base Solutions

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Making Dilutions From Acid and Base Solutions

Objective

The aim of the first part of the experiment was to make up dilutions
from acid and base solutions and record the results after adding
various indicators. The aim of the second part of the experiment was
to determine the pH values of the dilutions using both pH paper and a
pH metre recording the results for both tests.

Procedure - Part 1

For the first part of the experiment the correct amount of solid NaOH
needed to prepare 250 cm3 of a 0.1M solution was calculated and then
weighed out in a weighing bottle using balanced scales. The weight of
the empty bottle was recorded, minus lid before adding approximately
1g of solid NaOH, re-weighing the bottle plus contents and the new
weight recorded.

The Solid NaOH was then transferred to a 200 cm3 beaker where
approximately 150 cm³ of H2O was added to it and the solution stirred
with a glass stirrer (a magnetic stirrer was not available as
specified in the laboratory manual) until the solid was completely
dissolved. The solution was then transferred, via funnel, into a 250
cm³ volumetric flask and H2O added to increase the volume to 250 cm³,
the solution was then thoroughly mixed by inversion, ensuring the
stopper was held firmly in place.

Next a 1/10 dilution was prepared from the solution using volumetric
flasks and pipettes, although because the original solution was a 0.1M
concentration it was actually already a 1/10 dilution of a 1M
concentration making the 1/10 dilution prepared a 1/100 dilution of a
1M concentration. Therefore the two dilutions of NaOH were 1/10 and
1/100 of a 1M concentration both clearly labelled.

Dilutions of 1/10 and 1/00 using labelled volumetric flasks and
pipettes were then made up from a pre-prepared 1M solution of both HCl
and CH3COOH (adding the acid to water, not the other way around).

Nine test tubes were set out and numerically labelled 1-9 and a
pipette used to transfer a); 5 cm³ of 1/100 HCl into test tubes 1-3

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Related Searches

b); 5 cm³ of 1/100 CH3OOH into test tubes 4-6 and c); 5 cm³ of 1/100
NaOH into test tubes 7-9. The test tubes were then made into “sets”
compiled of one of each of the original sets and 1 drop of three
indicators added to then as shown below, the colour changes were then
recorded.

1 drop of methyl orange to each test tube in set 1 (test tubes 1, 4,
7)

1 drop of methyl red to each test tube in set 2 (test tubes 2, 5, 8)

1 drop of phenolphthalein to each test tube in set 3 (test tubes 3, 6,
9)

Procedure – Part2

An aliquot ~100 cm³ of each of the three concentrations (1M, 1/10M and
1/100M) of both acids HCl and CH3COOH were transferred into labelled
250 cm³ reaction flasks and the two concentrations (1/10M and 1/100M)
of the base NaOH into two labelled reaction flasks using pipettes. All
eight solutions were then tested using both pH paper and a freshly
calibrated pH meter to determine their pH values and the results
recorded.

Results

Table of results for part 1 of the experiment, Dilutions of acids and
base their reaction with indicators.

Solution

Reaction with methyl orange

Reaction with methyl red

Reaction with phenolphthalein

Set 1 – (1, 4, 7)

1. 5 cm³ 1/100 HCl

Pale Pink

4. 5 cm³ 1/100 CH3COOH

Pale Orange

7. 5 cm³ 1/100 NaOH

Orange

Set 2 – (2, 5, 8)

2. 5 cm³ 1/100 HCl

Very Pale Pink

5. 5 cm³ 1/100 CH3COOH

Pink

8. 5 cm³ 1/100 NaOH

No colour

Set 3 – (3, 6, 9)

3. 5 cm³ 1/100 HCl

No Colour

6. 5 cm³ 1/100 CH3COOH

No Colour

9. 5 cm³ 1/100 NaOH

Bright pink

Table of results for the experiment part 2, pH values of acids and
base, using pH paper and pH meter.

Aliquot ~100 cm³ of Solution

pH using pH paper

pH using pH meter

1M HCl

pH 1

pH 0.57

1/10M HCl

pH 2

pH 0.91

1/100M HCl

pH 4.5

pH 1.61

1M CH3COOH

pH 4

pH 2.30

1/10M CH3COOH

pH 5

pH 2.85

1/100M CH3COOH

pH 6

pH 3.30

1/10M NaOH

pH 11

pH 12.72

1/100M NaOH

pH 11

pH 11.70

Calculations

In this section I have included all the calculations and data used
during the experiment.

1) Calculations and data used to calculate how much solid NaOH needed
to make 250 cm³ of a 0.1M of NaOH.

Mass = mols × RM

RM = 23.00+16.00+1.00 = 40g

0.1 × 40 = 4g per dm³

1 dm³ = 1000 cm³ so to find how much in 250 cm³ need to 1000 cm³/4g
= 1g

1g of solid NaOH in a 250 cm³ 0.1M solution.

2) Weight of empty weighing bottle minus lid = 11.7956g

Weight of solid NaOH = 1.0056g

Total Weight =
12.8012g

3) Example of calculations of dilutions

1/10 NaOH → 25 cm³ NaOH in 225 cm³ H2O = 1/10 (or in this case 1/100
as the solution

0.1M was already to 1/10)

Discussion

The results from this experiment show that some errors have occurred
as the recorded data is not correct, compared with what we know should
have happened.

The tables below show what colour the three indicators should have
turned when added to the dilutions according to their pH range and
compares that with the recorded results.

1 2 3 4 5 6 7 8 9
10 11 12 13 14

[IMAGE]

pH range of indicators with acids – bases.

(Ref. Lewis and Evans, (1997), Chemistry 2nd edition. pg 302. Palgrave
Macmillan, UK).

Indicator

Lower pH colour

pH Range

Higher pH colour

Methyl Orange

Deep/Bright Red

3.2 ― 4.4

Bright/Yellow

Methyl Red

Bright/Deep Red

4.8 ― 6.0

Bright/Yellow

Phenolphthalein

No Colour

8.2 ― 10.0

Bright Pink

Comparison between recorded results and proven scientific data using
the pH results

from the pH meter as it is more accurate than using pH paper.

Solution

Ph value

From Ph meter.

Reaction with methyl orange

Reaction with methyl red

Reaction with phenolphthalein

What the reaction should have been

Set 1 – (1, 4, 7)

1. 5 cm³ 1/100 HCl

pH 1.16

Pale Pink

Red

4. 5 cm³ 1/100 CH3COOH

pH 3.30

Pale Orange

Reddish orange

7. 5 cm³ 1/100 NaOH

pH 11.75

Orange

Yellow

Set 2 – (2, 5, 8)

2. 5 cm³ 1/100 HCl

pH 1.16

Very Pale Pink

Red

5. 5 cm³ 1/100 CH3COOH

pH 3.30

Pink

Red

8.5 cm³ 1/100 NaOH

pH 11.75

No colour

Yellow

Set 3 – (3, 6, 9)

3. 5 cm³ 1/100 HCl

pH 1.16

No Colour

No Colour

6. 5 cm³ 1/100 CH3COOH

pH 3.30

No Colour

No Colour

9. 5 cm³ 1/100 NaOH

pH 11.75

Bright pink

Bright Pink

A base solution produces a greater concentration of HO- ions than
concentration of H3O+ ions and an acid solution produces a greater
concentration of H3O+ ions than concentration of HO- ions.

The comparisons show that only set 3 results, using phenolphthalein,
were correct. Phenolphthalein as an indicator reacts with colour
change to bases but not with acids. Basic solutions turn pink which
intensifies has base strength increases and remains transparent in
acidic solutions, therefore showing NaOH to be a strong base the
others to be acids. The results for the methyl red and orange
indicators were incorrect, a freshly calibrated pH meter was used and
so this indicates that the error/s occurred with the amount of
indicator used, the calculating and measuring of the dilutions or
there was contamination due to non sterile equipment.

If the methyl indicators had reacted with the dilutions as expected
then it would have shown CH3COOH to a weak acid and HCl to be strong
acid.

Conclusion

The experiment was partially successful. More attention to detail and
double checking of equipment to avoid contamination could have been
applied to ensure more a accurate result e.g. If the experiment was
carried out by an individual as apposed to a group then measurements
may have been more precise resulting in a more accurate overall
outcome.
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