volumetric analysis Flashcards
How does a chemist accurately measure the amount of a substance present in a sample of a solution.
It is essential for a chemist to be able to accurately measure the amount of a substance present in a sample of a solution. This is often done by titrating it against a standard solution.
Stages in quantitative analysis
In general, quantitative analysis follows the following steps:
1.
Accurately measure out a specific mass or volume of the chemical to be tested.
2.
If the sample is a solid, dissolve it in a known volume of water.
3.
Measure a property of the solution quantitatively, eg by reacting an unknown acid with an alkali of known concentration until the solution is neutral.
4.
Analyse the results to calculate the required measurement of the unknown substance (eg concentration of the unknown acid).
5.
Repeat the measurement using another sample to check that the results are consistent/reliable.
6.
Estimate the degree of uncertainty in the results.
This is often done by titrating it against a standard solution.
What are the 6 Stages in quantitative analysis
In general, quantitative analysis follows the following steps:
1.
Accurately measure out a specific mass or volume of the chemical to be tested.
2.
If the sample is a solid, dissolve it in a known volume of water.
3.
Measure a property of the solution quantitatively, eg by reacting an unknown acid with an alkali of known concentration until the solution is neutral.
4.
Analyse the results to calculate the required measurement of the unknown substance (eg concentration of the unknown acid).
5.
Repeat the measurement using another sample to check that the results are consistent/reliable.
6.
Estimate the degree of uncertainty in the results.
How do you make a standard solution?
A standard solution is one that has been made by dissolving an accurate mass of reactant into a known volume of water. This means that the concentration of the standard solution is accurate - so it can be used to work out the concentration of another solution that it reacts with.
Concentration can be measured in what ?
Concentration can be measured in g/dm3.
To make 250 cm3 of a standard solution with a concentration of 100 g/dm3, you need to:
1.
Accurately measure 25.0 g of the solid sample in a beaker.
2.
Add 150 cm3 of distilled water and stir the mixture using a glass rod until the solid has completely dissolved.
3.
Use a funnel to transfer the contents of the beaker into a 250 cm3 volumetric flask.
4.
Rinse the beaker and glass rod thoroughly with distilled water and pour the rinsings into the volumetric flask.
5.
Carefully add distilled water to the volumetric flask until the bottom of the meniscus is level with the horizontal line on the neck of the flask.
6.
Holding the stopper into the neck of the flask, carefully turn the flask upside down several times to thoroughly mix the solution.
A volumetric flask with a capacity of 100 cm3. Note the horizontal line around the neck of the flask which indicates the level for filling
What are the 6 steps to make 250 cm3 of a standard solution with a concentration of 100 g/dm3,
To make 250 cm3 of a standard solution with a concentration of 100 g/dm3, you need to:
1.
Accurately measure 25.0 g of the solid sample in a beaker.
2.
Add 150 cm3 of distilled water and stir the mixture using a glass rod until the solid has completely dissolved.
3.
Use a funnel to transfer the contents of the beaker into a 250 cm3 volumetric flask.
4.
Rinse the beaker and glass rod thoroughly with distilled water and pour the rinsings into the volumetric flask.
5.
Carefully add distilled water to the volumetric flask until the bottom of the meniscus is level with the horizontal line on the neck of the flask.
6.
Holding the stopper into the neck of the flask, carefully turn the flask upside down several times to thoroughly mix the solution.
A volumetric flask with a capacity of 100 cm3. Note the horizontal line around the neck of the flask which indicates the level for filling
How do you perform a titration? What are the 8 steps?
To work out the concentration of an acid, it can be titrated against a standard solution of an alkali.
Follow these steps:
1.
Fill a burette with the standard alkali solution. Run a little through the tap to make sure that no air is trapped. The zero line is at the top of the burette. Ensure that the bottom of the meniscus is level with the zero line.
2.
Use a pipette to measure exactly 25.0 cm3 of the unknown acid and place this into a conical flask.
3.
Add two drops of a suitable indicator (eg phenolphthalein or methyl orange). The indicator will show its acidic colour.
4.
Open the tap on the burette to start adding the alkali to the conical flask. Swirl the mixture in the conical flask continuously to mix it.
5.
As the colour of the indicator starts to change, slow down the flow of alkali from the burette so that it is dripping into the conical flask one drop at a time.
6.
When the indicator changes colour permanently, the end point has been reached.
7.
Record the volume of alkali used to neutralise the acid.
8.
Rinse out the conical flask, top up the alkali in the burette and then repeat steps 2 to 7 to ensure that the results are reliable.
Can a similar procedure can be used to work out the concentration of an alkali?
yes
How do you make sure titration results are reliable?
A titration should be repeated several times to ensure that the results are reliable (consistent). These results should then be averaged.
How do you evaluate titration results? (3 factors to bear in mind)
When evaluating experimental results, it is important to consider the accuracy, precision and validity of the measurements.
What does accuracy show? .
When evaluating experimental results, it is important to consider the accuracy, precision and validity of the measurements.
Accuracy describes how close a result is to the true value.
Precision is a measure of the spread of the measured values. If there is a big spread, the uncertainty is large.
If an experiment has a good degree of accuracy and the results have a small uncertainty, and the experiment is not flawed in any other way (eg an unfair test), then the results are valid
how close a result is to the true value.
What is Precision a measure of ?
the spread of the measured values. If there is a big spread, the uncertainty is large.
When are the results are valid of an experiment?
If an experiment has a good degree of accuracy and the results have a small uncertainty, and the experiment is not flawed in any other way (eg an unfair test), then the results are valid
How can you work out uncertainty ?
We can work out the uncertainty using a series of titration results:
The average is calculated by adding the values together and dividing by the number of values . The percentage error is calculated by dividing the uncertainty by the average and then multiplying by 100:
The concentration of a solution can be calculated using what formula?
For example, calculate the concentration when 10 g of NaCl is dissolved in 250 cm3 of water.
Concentration = 10 x 1000/250
Concentration = 40 g/dm3
The mass of solute dissolved in a solution can also be calculated by rearranging the equation:
Concentrate = mass divided by volume
The concentration of a solution can be calculated using the following formula.
For example, calculate the concentration when 10 g of NaCl is dissolved in 250 cm3 of water.
Concentration = 10 x 1000/250
Concentration = 40 g/dm3
The mass of solute dissolved in a solution can also be calculated by rearranging the equation:
The mass of solute dissolved in a solution can also be calculated by>
The concentration of a solution can be calculated using the following formula.
For example, calculate the concentration when 10 g of NaCl is dissolved in 250 cm3 of water.
Concentration = 10 x 1000/250
Concentration = 40 g/dm3
The mass of solute dissolved in a solution can also be calculated by rearranging the equation:
Mass= concentration x volume divided by 1000
rearranging the equation:
