Titration with HCl and NaOH

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Essay, Pages 3 (750 words)

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Aim: Determine the concentration of a solution of sodium hydroxide solution using a standardised solution of hydrochloric acid.

Health and safety:

HCl/NaOH -corrosive-avoid spills, mop-up (gloves)

-safety glasses

-laboratory coat

Pippete fillers- take care when using in case they snap when putting on the pipette

Indicator -stains/poisonous

Method

Step 1. Wash out:

i.) the pipette, with a little of the sodium hydroxide solution

ii.) the burette, with a little of the hydrochloric acid solution

iii.) the conical flask, with a little distilled water.

Step 2. Fill the burette with the hydrochloric acid solution, running some of the solution through the tap, until the bottom of meniscus is just on the zero level.

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Ensure that there is no air bubbles trapped.

Step 3. Using the pipette filler place 25cm3 of the sodium hydroxide solution in the clean conical flask.

Step 4. Add 4 or 5 drops of methyl orange indicator to the contents of the flask. Note the colour.

Step 5. Using the tap on the burette run the acid into the solution of sodium hydroxide in the conical flask (shaking the flask as the acid drips in) until the solution changes colour.

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At this point note the reading on the burette and the colour of the indicator. This first titration is 'rough' so only an approximate volume of acid is required.

Step 6. Repeat steps 2, 3 and 4

Step 7. This time:

i.) Run the acid into the solution until the volume of HCl is to within 2 cm3 of what it was in the rough titration (step 5).

ii.) At this point run the acid into the solution at a slower rate (dripping it in slowly and shaking the solution in the process) so you can read the volume of HCl required to neutralise the solution to one drip (0.05 cm3).

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Note: take an accurate reading to within 0.05 and take the reading at the first colour change in the solution.

Step 8. Repeat steps 6 and 7 Until satisfactory results are obtained:

* Two results are obtained within 0.05 of each other

* Three results within 0.10 of each other

Results

For 25cm3 of NaOH the following results were obtained by titration using HCl.

Rough

Accurate

Burette Reading end (cm3)

24.15

24.10

24.30

24.10

Burette Reading beginning (cm3)

0.00

Volume of HCl added (cm3)

24.15

24.10

24.30

24.10

Average Titre (cm3)

24.10

Calculations

1. Volume of HCl used = 24.10 cm-3

2. Concentration of HCl used = 0.107moldm-3

3. Volume of NaOH = 25.00 cm-3

4. Concentration on NaOH= ? moldm-3 or ? g moldm3

Calculations:

i.) Moles of HCl = 1.08 x 24.1 = n = 0.00260 moles

1000

ii.)Moles of NaOH= 0.0026 moles

NaOH + HCl NaCl + H2O

Ratio of substances 1 : 1

Number of moles 0.0026: 0.0026

iii.) Concentration: n = qV

1000

0.0026= (q x 25)/ 1000

q = (1000 x 0.0026)/ 25

q = 0.104 moldm-3

iv.) Concentration in g moldm-3: n = m

m = 0.0026 x 40 = 41.6g

Concentration on NaOH= 0.104 moldm-3 or 41.6 g moldm3

Compared to target value

Compare your result for the volume HCl required to neutralise the Sodium Hydroxide with the target value of 24.4 mol dm-3.

Your experimental value = 24.1 mol dm-3

Calculate the difference between your value and the target value as a % of the target value.

Percentage difference between my value and the target value = 1.23%

0.3 x100 = 1.23%

24.4

Errors in Titration Apparatus

By calculating the total percentage error in your apparatus you can compare this with how far you were off your target value to see how much of this is down to human error.

Error in volume from a pipette = 0.15cm3

Error in volume from a burette= 0.10cm3

Calculate the % error in the volumes you used in each of these apparatus:

Being 0.15 x 100 for the pipette

25.00

And 0.10x100 for the burette

24.1(V of HCl)

i.) % error in pipette volume = 0.60%

ii.) % error in burette volume = 0.43%

iii.) Total % error of apparatus = 1.03 %

We can now compare the two values we had for:

1.) percentage off target value (1.23%)

2.) percentage error in apparatus (1.03%)

If (1) < (2) that's as close as you can get with this apparatus

If (1) > (2) then there are some other things that you need to do to improve your technique. This is human error.

The suggested improvements in the accuracy of my experiment may reduce the margin for human error in my experiment.

Use a more accurate burette which measures to 0.01 this will mean rounding to 2 d.p. is more accurate. It will mean (when estimating between the lines marked on the burette every tenth of a cm3) instead of being 0.01 (estimating hundredths) from the actual value we would be 0.001 (estimating thousandths) thus our reading would be more accurate when we round it up to hundredths.

Repeat the titration more times- by doing this and averaging the closest few I would be more likely to find the precise volume of HCl required to neutralise the solution.