Post by RGBM211

ermm i found this FOR CHEMISTRY PAPER 5 on studyguide.pk looks extremly helpful.... here yu go

guys....

For planning section:

When asked to draw a diagram, ALWAYS mention the volume of the apparatus being used.

The one most often used is gas syringe. I think it is better to limit the volume of syringe to

less than 500 cm3 . Also, the volume of a commonly used small test tube is around 16 cm3

and that of a boiling tube is about 25 cm3. We always have to keep the volume of the

apparatus we are using while choosing the volume of the solution for the planning. Saying

that we are going to place 50cm3 of aqueous HCl in a test tube is of course not going to

please the examiners. I think I ended up doing the same in one of the papers where we had

to prepare different concentrations of a given solution. I chose volume of water to be added

to a beaker to be 250 cm3. However, the ER stated that the volume should be less than

200cm3, that so being since the volume of a commonly used glass beaker is around 250cm3.

Someone carrying out my expm would have a fully filled beaker. However, we can use

greater volume if we state that the volume of the beaker is 400cm3 or 500 cm3 or so on. So,

stating the volume of the apparatus is very very important

Always while measuring volume of solutions, use burette or pipette because they have low

PERCENTAGE error. The question of percentage errors are usually asked in P5. The

percentage error becomes very unacceptable if we are measuring SMALL volumes of

solution or small masses. So, a 3dp balance is much better for measuring masses than a 2dp

balance as it would have much less % error when small masses are being measured

Shared by zeebujha

Okay guys, just over with P5. Let me put down whatever I understood from the P5s of M/J 2002 to

O/N 2010 into words.

While heating crystals strongly, do not use a simple test tube or any apparatus with sharp

edges as they are liable to crack at the sharp edges. For very strong heating to a constant

mass, a crucible placed on a pipe clay triangle is appropriate. And of course to measure the

mass of the crucible + solid, you would have to place it on a balance. But we wouldnt want

to fry our poor balance by placing a very hot crucible on top of it. So, allow the crucible to

cool for a few minutes by placing it on a heat mat.

When using a magnesium ribbon or any dirty surface, clean it with SANDPAPER

How to prepare crystals (which is also linked with solubility)

Many methods are mentioned in MS but I found this to be the simplest:

CIE usually demands us to prepare crystal after preparing a saturated solution of the crystal.

So, we have to prepare a saturated solution first:

1. Take a fixed volume of water in a beaker of appropriate volume

2. Add the crystal to the water and stir continuously. You have to allow some time for the crystal to dissolve as it is an equilibrium process

3. After 5 min of stirring , if no solid crystals appear, add further mass of crystal

4. And repeat the process until solid appears in the beaker

5. Filter the solution using a filter paper and funnel so that the saturated solution is collected in a beaker(whose mass has been measured previously) beneath the funnel

Now we have a saturated solution in a beaker.

How to get the crystals:

We have to place the beaker in a warm water bath. We could use a burner as well but

there is a risk of overheating the solution which could decompose the crystal. If the heat

is appropriate, the water of the solution should evaporate and we should have dry

crystals ready.

How to measure solubility:

6. Measure the mass of beaker + solution from step 5

7. Subtract the mass of beaker from the mass in 6 to get the mass of saturated solution

8. And evaporate as shown above to get the mass of crystals.

9. Measure the mass of crystal + beaker

10. Subtract mass of beaker from mass in 8 to get the mass of crystals

11. Subtract the mass of crystals from the mass of saturated solution to get the mass of

water in the solution

12. I have assumed that all the masses are in grams. So, to get the solubility: Mass of

crystal x 100/Mass of water

When you have to remove moisture from :

1. Surface:

Wash the surface with a stream of propanone. The water gets dissolved in the

propanone and repeat it multiple times. Then gently heat the surface to evaporate

the propanone from the surface

2. Vapour:

Use dessicants such as:

ANHYDROUS sulphuric acid

ANHYDROUS calcium chloride

Silica gel

You have to pass the vapour from the beaker containing the dessicant

Also, a useful property of soda lime is that it absorbs BOTH water vapour and carbon

dioxide

We are also regularly asked to measure enthalpy changes.

Most of us already know that we use a plastic cup and thermometer for this purpose

However this has many disadvantages (asked regularly) and here are some of them with the

required measures:

1. Heat loss to the surroundings from the beaker:

To avoid this:

a. Cover the plastic cup with a lid

b. Place the cup in a beaker. The air in the beaker acts a good insulator.

c. Use multiple cups so as to thicken the lateral layer of plastic

2. Instability of the cup

a. Place the cup in a glass beaker

3. For exothermic reactions, spray of the solution very likely

a. Use a large beaker to carry out the experiment rather than the small plastic cup

(which has a small volume)

b. Put a lid on the top of the beaker (this only MINIMISES the spray, doesnt prevent it

totally)

When we are heating a volume of water in glass beaker, there are two cases of heat loss to consider:

a. Heat loss from the beaker of water

b. Heat loss from the burner heating the volume of water

When preparing a solution of a fixed concentration from a given parent solution of concentration ,

say, 2.0 mol/dm3

We are required to prepare a solution of concentration of 1.0 mol/dm3 with volume 250

cm3 (the concentration cannot be greater than 2 !!!!!!!!!!!!!!)

a. Add 125 cm3 of the parent solution to a volumetric flask of marking at 250 cm3

[1/2 x 250 = 125 cm3]

b. Top off with water to the mark of 250 cm3. Use a wash bottle for this purpose to

have greater control over the addition process

When required to prepare 250 cm3 of 1 mol/dm3 solution of a crystal of Mr 50g

1. First realize that we need only 250 cm3, not 1 dm3

2. Now in 250 cm3 there will be: 50/4 = 12.5 g of the solid

3. So, first add 12.5 g of solid to 50 cm3 of water in a BEAKER ( not the volumetric flask yet).

4. Also note that the volume of water is less than 250 cm3.

5. Stir properly and if the solid doesnt dissolve add more water until it fully dissolves

6. Then transfer the solution from the beaker to a volumetric flask. Remember to rinse the

beaker with water and transfer the solution to the volumetric flask

7. Stopper the flask and shake properly

8. Finally, add the required volume of water to make the solution upto 250 cm3 USE A FUNNEL

OR YOU RISK OVERSHOOTING THE MARK

Titration is accurate because:

1. Standard solution of acid/base is used

2. we obtain concordant titres

3. % error in pipette and burette is very small

4. The end point of a titration is sharp

For Filtration:

1. If you require a great separation , use a fluted filter paper

2. If you require a quick filtration, use vacuum filtration

Distillation:

1. Distillation is a great way of determining boiling point of a liquid

2. Melting point can be measured using a Mel Temp

Some Random points:

We do not need to measure the mass of a reagent or volume of reagent that is in

excess

If you want to flush oxygen out of a system, use an inert gas that doesn't oxidise .

Very useful when we are conducting a reduction experiment of metal oxides

If you want to collect water vapour as water droplet, collect the vapour in a beaker

placed in ICE BATH

During magnesium and hydrochloric acid experiment, to avoid loss of gas do not

start the experiment by dropping a piece of Mg in the container of acid!!!

Rather, use a divided flask (I would appreciate it greatly if someone explains what

that actually means) or place a test tube containing the magnesium metal in the

solution and pull the string to start the reaction

If a syringe gives wrong volume, it could be because the syringe got stuck during the

experiment

If the percentage difference between the measured value and the true value is more

than the maximum apparatus error, this means the experimenter's technique needs

modification. If not, the error could be considered to be entirely due to the intrinsic

error or the apparatus used

if the experiment involves plotting a pH against volume curve, the accuracy of the

curve can be increased by using data logger interface and computer combined

together to plot the graph

For Question 2:

For question 2, explanation in O/N 2010 ER is the best ever:

The ER states:

Two points about data tables are common every session. Firstly, candidates need to label

each new column used. These labels require three items a written label, a correct unit and

an expression to be used to calculate the column data. All of these items are required for

credit to be awarded (often the expression is omitted). Secondly, the data needs to be

correctly quoted, to either a specified number of decimal places or significant figures. In this

examination, the data requirement was for 3 significant figures. Usually, the table head and

data requirements are given below the table, so candidates should be advised to look there

before calculating any data.

In scaling the axes on a graph plot, candidates should always consider if the true origin (0,0)

would be a point on the line. If so, it should always be included in the scales since it would

always be a reliable, correct point. Two factors need to be taken into account when scaling

the axes. The points need to be plotted within at least half of each axis and the scale should

be reasonable to facilitate accurate plotting. For example data of 1, 2, 4 or 5 to 1 large

square (of 10 small squares) would be reasonable but 3 or 6 or 7 to 1 large square (of 10

small squares) would not. Some candidates would benefit from practice in fitting a

reasonable scale into the grid first that uses at least half of each axis.

When referring to the reason for an anomalous result, it should be stressed to candidates

that they unambiguously define the point (e.g. Experiment 4) before stating a reason. This is

vital when there is more than one anomaly.

For tables:

Each column heading needs three things, a description (e.g. concentration of acid in water),

a unit (e.g. /mol dm-3) and an expression to calculate the data (e.g. B x 0.1/20) as required in

the text below the table.

Also below the table is stated a requirement for a specific number of significant figures or

decimal places. There is a general misunderstanding of the difference between significant

figures, decimal places and trailing zeros. In this case the requirement was for three

significant figures. There was a frequent misunderstanding of this requirement. For example,

0.0105 (3sig figs.) was given as 0.011 (3dp), even to the extent of seeing 0.0105 written but

crossed out and replaced by 0.011 where, presumably, the candidate believes three decimal

places is the same as three significant figures. Also a calculated value of 0.05 needs to

reported as 0.0500 if three significant figures are required. Care needs to be taken with the

reporting of data. The two required columns involved a great deal of calculation. Each

column required the calculation of moles of sodium hydroxide, converting this into moles of

succinic acid then a concentration of acid. Only the more able candidates had any success

here.

Many managed two parts of these calculations but usually not the 2:1 relationship between

succinic acid and sodium hydroxide. This may have been due to unfamiliarity with succinic

acid or its formula. Quite a significant minority labelled the columns moles of sodium

hydroxide rather than concentration in each solvent even though this was stated below the

table.

For graph plotting:

The scaling of the axes seems to cause misjudgement in the minds of some candidates.

There seems to be a determination to cover the whole span of the grid regardless of the

implications of the axis scaling necessary to achieve that. An example of difficult scaling is 1

large square being 0.03. This would make each small square (1 mm) 0.003.

This creates difficulties for the candidate in correctly plotting data of the nature of 0.00552

so mis-plotting is often seen when such inconvenient scales are chosen. Another example of

difficult scaling was a 1 mm square being 0.00218 on the x-axis and 0.000223 on the y-axis.

Such scales require the extensive use of a calculator to adequately plot the data which in the

very least may take some time. Some candidates chose a scale for the acid concentration in

water that did not accommodate all the points giving the last point outside the grid.

The scaling chosen must adequately cover the available grid but the minimum requirement

is for the plot to cover at least half of the grid in both directions. So the candidates must

make a compromise between scaling and grid coverage to use at least half of the grid in both

directions with a reasonable scale that can be easily used.

Much better scaling for the concentration in the water layer would have been 1 large

square for 0.0125 or 0.02 mol dm-3 (depending on which axis) and for the concentration in

the ether layer, 1 large square for 0.001 mol dm-3.

Two more factors need to be considered in graph work. Firstly it is the independent variable

that is to be plotted on the x-axis and the dependent on the y-axis. Almost uniquely in this

particular question either variable could be plotted on either axis. Secondly the

data/relationships should always be examined to see if the origin (0,0) is a valid point. If so

(as in this question) then the scaling used must include the origin and any line drawn would

include the origin since it would be a definite point not subject to any experimental error as

the other points may be.

A requirement for determining a slope is the use of construction lines on the plot to produce

co-ordinates for the slope calculation. Many candidates did not draw construction lines.

Hope all this helps with question 2

Titration:

Dont forget to:

- rinse the burette with acid

- rinse the pipette with alkali

- When emptying the pipette into the conical flask, allow it to empty under gravity, and then

touch the surface of the liquid with the pipette for approximately one second

- remove funnel before titrating

- add only two drops of indicator

- swirl mixture during titration

- Wash sides of the flask regularly using a wash bottle

- titrate dropwise near end-point

- read the burette accurately (eyes horizontal, bottom of meniscus)

- record burette reading to 2 decimal places (second 0 or 5)

For better observation:

1. Place a white tile under the conical flask

2. Illuminate the burette while taking the reading

Some more random points:

If asked in an enthalpy experiment what effect does incomplete value of ethanol

have on the measured enthalpy change of combustion:

1. Value decreased

2. Because less C=O bonds formed

Errors in measuring enthalpy change:

1. Maximum temperature difficult to determine (Measure temperatures often or

_ use a computer and datalogger to measure the temperature)

2. The temperature of the solution is not uniform (Stir the solution throughout the l

_ experiment or use a magnetic stirrer)

Also, even if there are no experimental error while measuring the enthalpy change

of combustion of ethanol , we cannot find the accurate value of the enthalpy. The

defintion of enthalpy requires formation of liquid water on combustion but in our

experiment the water is being turned into vapour!!!!!!!!

When asked "Which equipment should be changed so that the accuracy is improved

the most?", always choose the equipment which has the highest percentage error in

the experiment.

The volume of a polystyrene cup used in the lab is roughly 150 cm3