IGCSE & A/AS Levels students Forecast

AS & A Levels: Quantitative Chemistry part 1

THE MOLE

A mole of a substance is the amount of that substance that contains the same number of elementary particles as there are carbon atoms in 12.00000 grams of carbon-12.

One mole of carbon-12 has a mass of 12.0g.

One mole of hydrogen atoms has a mass of 1.0g.

One mole of hydrogen molecules has a mass of 2.0g.

One mole of sodium chloride has a mass of 58.5g.

The number of particles in one mole of a substance is 6.02 x 10²³. This is known as Avogadro's number, L.

 Thus when we need to know the number of particles of a substance, we usually count the number of moles. It is much easier than counting the number of particles.

 

The number of particles can be calculated by multiplying the number of moles by Avogadro’s number. The number of moles can be calculated by dividing the number of particles by Avogadro’s number.

The mass of one mole of a substance is known as its molar mass, and has units of gmol^-1. It must be distinguished from relative atomic/molecular/formula mass, which is a ratio and hence has no units, although both have the same numerical value.

The symbol for molar mass of compounds or molecular elements is Mr. The symbol for molar mass of atoms is Ar.

Mass (m), molar mass (Mr or Ar) and number of moles (n) are thus related by the following equation:

Mass must be measured in grams and molar mass in gmol^-1.

REACTING MASSES

It is possible to use the relationship moles = mass/m_r to deduce the masses of reactants and products that will react with each other.

When performing calculations involving reacting masses, there are two main points which must be taken into account:

The total combined mass of the reactants must be the same as the total combined mass of the products. This is known as the law of conservation of mass.

The ratio in which species react corresponds to the number of moles, and not their mass. Masses must therefore all be converted into moles, then compared to each other, then converted back.

i)  reactions which go to completion

Eg What mass of aluminium will be needed to react with 10 g of CuO, and what mass of Al₂O₃ will be produced?

3CuO(s) + 2Al(s) à Al₂O₃(s) + 3Cu(s)

       10 g

= 10/79.5

= 0.126 moles of CuO

3:2 ratio with Al

so 2/3 x 0.126 = 0.0839 moles of Al, so mass of Al = 0.0839 x 27 = 2.3 g

3:1 ratio with Al₂O₃

so 1/3 x 0.126 = 0.0419 moles of Al₂O₃, so mass of Al₂O₃ = 0.0419 x 102 = 4.3 g

fabhamsy: Hawk and Heron

fabhamsy: Hawk and Heron:                                                      
Hawk: -Common they are some bird of prey. -Widely distributed and avarying greatly ...

Tata Tertib UN

Nilai Try Out 3

UN Timetable SMP 2014

AS & A Levels: Chromosomes part 1

A living organisms grow and reproduce. since organism

The nucleus contains chromosomes

Before a eukaryotic cell divides, a number of thread like structures called chromosomes gradually 

become visible in the nucleus. They are easily seen, because stain intensely with particular stains. they were originally termed chromosomes, because 'chromo' means colored and 'some' means body.

The number of chromosomes is characteristic of the species. such as, in human cells there are 46 chromosomes, and in fruit fly there are only 8 chromosomes.

• There are matching pairs of chromosomes called homologous pairs. Each pair is given a number.
• Human chromosome is 46 which means 23 from the father and 23 from the mother
• Sex chromosomes, which determine the sex of the individual. All other chromosomes are called autosomes.
• The pairs of chromosomes can be distinguished not only by size and shape, but because each pair has a distinctive banding pattern when stained with certain, as











Set of chromosomes in human  (Karyogram)





A Level: Enthalpy changes part 3

A Level: Enthalpy changes part 2

AS & A Levels: The Avogadro constant

IGCSE: Moles Concept part 1

IGCSE: Moles Extra Task

A Level: Enthalpy Changes part 1

 

A Level  chemistry 2/5/2014

Enthalpy Changes  Part 1

 students should:

1.     know that reactions can be endothermic or exothermic

2.     understand that enthalpy change (ΔH) is the heat energy change  measured under conditions of constant pressure

3.      know that standard enthalpy changes refer to standard conditions,   i.e. 100 kPa and a stated temperature (e.g. ΔH298)

4.      be able to recall the definition of standard enthalpies of combustion  (ΔHc ) and formation (ΔHf )

                   Exothermic and endothermic reactions

     If a reaction produces heat (increases the temperature of the surroundings)  it is exothermic

     If the temperature of the reaction mixture decreases (i.e. heat is absorbed) then the reaction is endothermic.

Exothermic a reaction which produces heat (ΔH has a negative value by convention, -ve)

Endothermic a reaction which absorbs heat (ΔH has a positive value by convention, +ve)

Enthalpy of reaction: The change in internal (chemical) energy (H) in a reaction   = ΔH. 

    The most stable state is where all energy has been released.

    When going to a more stable state, energy  will be released, and when going to a less stable state, energy will be gained (from the surroundings).

    On an enthalpy level diagram, higher positions will be less stable (with more internal energy)  therefore, if the product is lower, heat is released (more stable, ΔH is -ve) but if is higher, heat is gained (less stable, ΔH is + ve).

    

If

A Level: Chemistry Class Test

1.          Calculate the volume occupied by 2,50 mol of carbon dioxie at a pressure of 100 KPa and a temperature of 19 oC.

(R= 8,31 JK -1 mol -1

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2.             A flask volume 5,00 dm3 contained 4.50 gram of oxygen. Calculate the pressure exerted by the gas at a temperature of 250  oC.

(Rr = 8,31 JK -1 mol -1 , Mr Oxygen = 32.0)

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3.             Calculate the volume occupied by 272 g of methane at a pressure of 250 KPa and  a temperature of 55 0C

(R = 8,31 JK-1 mol -1); Mr methane = 16 .0)

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4.       Describe 4 the behaviors of liquid state?

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5.          Explain the following:

a.       Why most metals are the strong, but ionic solids are brittle?

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b.      Why is an alloy of copper and tin stronger than either copper or tin alone?

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6.          Crystals of sodium chloride have a lattice structure.

a.       Describe a sodium chloride.

b.      Explain the following properties of sodium chloride.

(i)                  Sodium chloride has a high melting point

(ii)                Sodium chloride conducts electricity when molten but not when solid

(iii)               Sodium chloride is hard but brittle

  

7.             Why purifying and remolding aluminium is much cheaper than extracting aluminium from bauxite ore?

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8.          What is ceramics?

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9.          Describe the terms alloy !

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10.         Why is brass stronger than pure copper?

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A Level: Ethanol Emulsion For Testing Of Fats

Materials:

• Ethanol
• Brownies
• Sticky Rice
• Bread
• Whole
  Bread
• White Bread
• Oil
• Water

Equipments:

• Evaporating Dish
• Test Tube
• Test Tube Rack
• Filter Funnel
• Filter Paper
• Pestle/Mortar
• Pipette
• Tongs

Preparations of a solid sample:

1. Completely crushes adequate amount of food with a clean and dry mortar and pestle.

2. Transfer an adequate amount of crushed food sample to a clean test tube

3. Adds 2cm ethanol above the food sample

4. Shakes the test tube thoroughly and safely .

5. Allows the solid to settle for about 3 min

6. Decants the ethanol carefully into a clean,dry test tube without transference of food particles.

7. Adds 2cm of de-ionized water to the test tube

8. Compares each result with the negative control

9. Compares the heights of the emulsion layer of all the samples

Process:

1. Add the food sample to 2 cm3 of ethanol, shake well.
2. Allow to settle in a test tube rack for 2 minutes for food to dissolve in ethanol.
3. Empty any clear liquid into a test tube containing 2 cm3 of distilled H2O.
4. A MILKY-WHITE EMULSION is a positive result: lipid is present.
5. If the mixture remains clear, there are no fats present in the sample

RESULTS

SUBSTANCE TESTED	PRESENCE OF SPOT	LIPIDS PRESENCE
Brownies	Yes	Yes
Rice	-	-
Sticky Rice	-	-
Cooking Oil	Yes	Yes
White bread	Yes
Whole bread	No	No

By: Nadia Hasna Karimah

Cell Membrane And Transport

Phospholipids

A lipid that containing phosphate group in its molecule. From phospholipids, little bags can be formed in which chemicals can be isolated from the external environment. These bags are the membrane-bound compartments that we know as cells and organelles.

The basic structure of a membrane is a 7 nm thick phospholipid bilayer with protein molecules
spanning the bilayer or within one or other layer. Phospholipids and some proteins move within the
layers. Hence the structure is described as a fl uid mosaic – the scattered protein molecules resemble pieces of a mosaic.

Phospholipid bilayers are a barrier to most water 

soluble substances because the interior of the membrane is hydrophobic.

Structure Of Membrane

Cholesterol is needed for membrane

fluidity and stability.

Some proteins are transport proteins, transporting molecules or ions across the membrane. They may be either channel proteins or carrier proteins. Channel proteins have a fixed shape; carrier proteins change shape. Some proteins act as enzymes – for example, in the cell surface membranes of microvilli in the gut. Glycolipids and glycoproteins form receptors – for example, for hormones or neurotransmitters. 

They also form antigens, which are cell recognition markers.

Transport Across The Cell Surface Membrane

Th e cell surface membrane controls exchange between the cell and its environment. Some chemical reactions take place on membranes inside cell organelles, as in photosynthesis and respiration. 

Diffusion is the net movement of molecules or ions from a region of their higher concentration to one of lower concentration. Oxygen, carbon dioxide and water cross membranes by diffusion through the phospholipid bilayer. Diffusion of ions and larger polar molecules through membranes is allowed by transport proteins. This process is called facilitated diffusion.

Active transport – the sodium–potassium pump

By: Nadia Hasna Karimah