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Investigating titrations

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Explainer Video

Tutor: Alexander

Summary

Investigating titrations

In a nutshell

This practical involves using the required apparatus to carry out a titration using accurate measurements and the correct technique. Using the results of a titration you can calculate the concentration of the unknown solution.

Equations

This equation forms the basis of this experiment:

word equation	Symbol equation
$hydrochloric\ acid\ +\ sodium\ hydroxide\ \rightarrow\ sodium\ chloride\ +\ water$	$HCl(aq)\ +\ NaOH(aq)\ \rightarrow\ NaCl(aq)\ +\ H_2O(l)$

These equations will be used as part of this experiment:

$Titre\ (cm^3)\ = \ end\ volume\ (cm^3)\ -\ start\ volume\ (cm^3)$

$\dfrac{titre\ 1\ +\ titre\ 2}{number\ of\ titres}\ =\ average\ titre$

Equipment list

The following equipment list is used to carry out a titration.

Equipment	Use
Burette	Used to dispense acid into the conical flask
Funnel	Placed in the burette to help pour the acid into the burette
Pipette	Measures a specific volume of alkali $(25\,cm^3)$ to be dispensed into a conical flask
Pipette filler	Used to fill the pipette
Conical flask	Where the titration occurs
Boss	Used to support the burette
Clamp	Used to support the burette
Stand	Used to support the burette
White tile	Placed under a conical flask to help observe a colour change from the indicator
Goggles	Protect eyes from safety hazards
Gloves	Protect hands / skin from safety hazards
Acid (hydrochloric, sulfuric or nitric), unknown concentration	Dispensed into alkali to calculate the concentration of acid
Sodium hydroxide, known concentration	Used to neutralise the acid
Indicator (methyl orange, phenolphthalein)	Gives a visual response of when the pH of a solution changes

Experiment: Determination of reacting volumes using a strong alkali and a strong acid via a titration

Experimental variables

The independent variable is the one you change. The dependent variable is the one which depends on what has been changed, therefore it is the one you measure. The control variable is the one which is kept constant.

Independent variable	$Volume\ of\ acid$
Dependent variable	$pH$
Control variable	$Volume\ of\ alkali$

Safety precautions

When performing experiments, it is very important you consider safety precautions. This is so you and others do not get hurt.

Hazard	Risk	Safety measure
Sodium hydroxide solution	Skin and eye irritation	Wear gloves and eye protection
Acid solution (hydrochloric, sulfuric, nitric)	Skin and eye irritation	Wear gloves and eye protection, fill burette slowly using a funnel

Method

Follow the steps below for the practical.

Chemistry; Chemistry practicals; KS4 Year 10; Investigating titrations

1.	Burette filled with acid (hydrochloric acid)
2.	Boss, clamp and stand
3.	Conical flask containing solution

1.	Use the pipette and pipette filler to add $25 \, cm^3$ of the alkali into a clean conical flask.
2.	Add a few drops of the selected indicator to the flask and place the flask on a white tile.
3.	Fill the burette with acid using a funnel. Make sure to remove the funnel to not skew the readings and note the starting volume.
4.	Add the acid slowly from the burette to the alkali in the conical flask. Make sure to swirl the solution before adding further acid and check for a colour change.
5.	Stop adding the acid when the end-point is reached (when the indicator gives a positive result). Note down the final volume reading of the burette. To ensure that more accurate results are obtained add in the acid drop wise near the end-point.
6.	Repeat steps 1 to 5 until you get concordant titres (see analysis for concordant titre definition).

Analysis

Record the results in a table as shown:

Repeat	Start volume $(cm^3)$	End volume $(cm^3)$	Titre $(cm^3)$
Rough	$12.80$	$0.00$	$12.80$
$1$	$20.05$	$7.60$	$12.45\ \checkmark$
$2$	$22.70$	$10.05$	$12.65$
$3$	$22.25$	$9.75$	$12.50\ \checkmark$

To calculate the titre use the following equation:

$Titre\ (cm^3)\ =\ end\ volume\ (cm^3)\ -\ start\ volume\ (cm^3)$

$\frac{titre\ 1\ + \ titre\ 2}{number\ of\ titres}\ =\ mean\ titre\ (cm^3)$

$mean\ titre\ = \frac{12.45\ +\ 12.50}{2}\ =\ 12.475 \, cm^3$

Readings should be to two decimal points, ending in zero or five (where liquid level is between graduations on the burette).

Make sure to mark your concordant results. Concordant titres are within $0.20 \, cm^3$ or less of each other. Calculate the mean titre by summing the titres and then dividing by the total number of titres.

Conclusion

Using the obtained measurements from the experiment you could calculate the concentration in $(mol/dm^3)$ of the acid.

Evaluation

What actions were taken to ensure accurate measurements? What improvements could be made to the method? Was the practical carried out safely?

Learn with Basics

Length:

Unit 1

Physical and chemical properties

Unit 2

The pH scale and indicators

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Optional

Unit 3