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Answers to the workbook

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What you need to know

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The leaves are the part of a plant where most photosynthesis takes place.
If you cut a leaf in half and look at the cut end you will see something like this:

1. Waxy cuticle, this gives the leaf a waterproof layer, which lets in light.

2. Upper epidermis - provides an upper surface.

3. Palisade cells, which are packed full of chloroplasts.

4. Spongy mesophyll. Collection of damp, loosely packed cells.

5. Lower epidermis is the layer of cells on the lower surface.

6. Air space inside the leaf, allows contact between air and moist cell surfaces.

7. Stoma, a hole in the leaf which gases diffuse through.

8. Guard cells, which change shape to close the stoma.

See the section on for more details on plant structure.

One amazing feature of leaves is that they have tiny holes in them to let the gases carbon dioxide and oxygen enter and exit. One of the holes is called a 'stoma', although just to be confusing, when there are more than one they are called 'stomata'.

A stoma is just a hole. It is controlled by two guard cells which change shape to either open or close the hole. Something makes water enter the cells by osmosis and so they swell up and change shape, but no-one is quite sure of the trigger.
Why do the swollen cells bend?
One side is thicker than the other, a bit like having a piece of tape on the side of long balloon and then blowing it up. Try this at home!

The stomata (air holes) on plants are normally open during the day and closed at night. Photosynthesis happens during the day.

Photosynthesis is the way that plants make their food using energy from sunlight.
What is the word equation for photosynthesis?
Plants use the green dye (or pigment) called chlorophyll to pick up the energy from the sunlight.

Plants make sugar and use some of it for energy to keep them alive ( respiration) but they also use some for growth and repair by making fats and proteins.

However, it is not always sunny so plants need to be able to store some ofthe sugar they make, so they convert it to a storage carbohydrate.
Why bother storing it as starch?
Plants could use starch or glucose. Starch is insoluble (it does not dissolve in water) while glucose is soluble. This means that if starch is used, less water is required to keep its food stored.
What things might effect how well plants can carry out photosynthesis?
The amounts of water, carbon dioxide, sunlight and temperature.

The amount of water is effected by how much is taken up through the roots and how much is lost from the leaves. If less water is available in the leaf then photosynthesis will occur more slowly.

Similarly, if there is less carbon dioxide around then photosynthesis will occur more slowly. There wont be enough of the fuel ( substrate) to get the reaction to work.

If there is less sun, which usually means it is cooler too, then there is less energy for photosynthesis and it occurs more slowly. So photosynthesis works best when it is warm and sunny - don't we all!
How could you measure how quickly photosynthesis is occurring?
You could wait ages and then see how much glucose is in the plant. Boring and slow!

Alternatively, one favourite way is to measure how much oxygen gas is given off. This can be done easiest by counting the bubbles of gas as they come off a water plant like the pondweed, Elodea.

When plants have their stoma open during the day, they can let carbon dioxide and oxygen diffuse through them. However, water can also diffuse out from the moist inside of the leaf to the drier air outside. This movement of water is called 'transpiration'?.

Transpiration is a bit like the sweat on our skin and may also help to cool the leaves. Large trees may lose nearly 200 litres of water a day by transpiration.

On a hot sunny day plants could become dehydrated as they lose water even though photosynthesis works best in these conditions. They cannot draw up water from their roots quickly enough so photosynthesis starts to slow down. A big problem!
So how do plants stop themselves drying out?
They close their stomata. This happens because the guard cells lose water too and go floppy. A very neat trick!

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Structure of leaves The leaves are the part of a plant where most photosynthesis takes place. If you cut a leaf in half and look at the cut end you will see something like this: /**/ Key: Waxy cuticle, this gives the leaf a waterproof layer, which lets in light.

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Do not forget about photosynthesis for the mock

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Photosynthesis - AQA - Revision 1 - GCSE Combined Science - BBC Bitesize

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Revise how plants make their own food using photosynthesis for GCSE Combined Science, AQA.

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Reactions and moles - Higher - Calculations in chemistry - Higher - AQA - GCSE Combined Science Revision - AQA Trilogy - BBC Bitesize

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Learn about and revise calculations in chemistry for Higher tier students with this BBC Bitesize GCSE Combined Science (AQA) study guide.

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Not a pretty website but great content

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Empirical formula and formula mass from reacting masses (easy start, no moles!) 5.

The EMPIRICAL FORMULA of a compound can be worked out by knowing the exact masses of the elements that combine to form a given mass of a compound.

The e mpirical formula of a compound is the simplest whole number ratio of atoms present in a compound. ( see section 3. for some simpler examples ). Here the word ' empirical' means from experimental data.

Do not confuse with molecular formula which depicts the actual total numbers of each atom in a molecule.

The molecular formula and empirical formula can be different or the same.

They are the same if the molecular formula cannot be simplified on a whole number basis.

Examples where molecular formula = empirical formula

e.g. for sodium sulfate Na₂SO₄ and propane C₃H₈

You cannot simplify the atomic ratios 2 : 1 : 4 or 3 : 8 to smaller whole number (integer) ratios

Similarly for pentane, the diagram shows it's molecular formula is C₅H₁₂, which cannot be simplified, so the molecular formula = empirical formula.

Examples of where molecular formula and empirical formula are different e.g.

butane molecular formula C₄H₁₀, empirical formula C₂H₅

numerically, the empirical formula of butane is 'half' of its molecular formula

4 : 10 ==> 2 : 5

glucose molecular formula C₆H₁₂O₆, empirical formula CH₂O

numerically, the empirical formula of glucose is ' ¹/ ₆ th' of the full molecular formula

6 : 12 : 6 ==> 1 : 2 : 1

Suppose you start with a molecular structure like octane (left diagram)

If you count the atoms you find the molecular formula is C ₈H ₁₈.

BUT the simplest ratio formula, that is the empirical formula is 'half' of the molecular formula i.e. C ₄H ₉, but remember, if you have to suggest a structure for a molecule you must know at least its molecular formula.

AND don't forget to be able to think, calculate and deduce the other way round e.g.

(i) Suppose a hydrocarbon molecule has an empirical formula of C ₂H ₅ and a molecular mass of 58 (C = 12, H = 1).

Deduce its molecular formula. The empirical formula mass = (2 x 12) + 5 = 29.

Dividing 58 by 29 gives 2. So the molecular formula is 2 x the empirical formula = C₄H₁₀

(ii) Suppose a molecule has an empirical formula of simply CH, but a molecular mass of 78 (C = 12, H = 1).

The empirical formula mass is 12 + 1 = 13. Therefore 78/13 = 6, so the molecular formula is 6 x CH = C₆H₆

Where the empirical formula and molecular formula are different, you need extra information to deduce the molecular formula from the empirical formula (see link below).

This page is only concerned with calculating empirical formula.

For more advanced students see Using moles to calculate empirical formula and deduce molecular formula of a compound/molecule (starting with reacting masses or % composition)

The following examples illustrate the ideas using numbers more easily appreciated than in real experiments.

In real laboratory experiments only a fraction of a gram or a few grams of elements would be used, and a more 'tricky' mole calculation method is required than shown here ( dealt with later for higher students in section 8 ).

However the examples below show in principal how formulae are worked out from experiments.

Any calculation method must take into account the different relative atomic masses of the elements in order to get to the actual ratio of the atoms in the formula.

For example, just because 10g of X combines with 20g of Y, it does not mean that the formula of the compound is XY 2 !
If you divide the mass of each element by its atomic mass, you actually get the atomic ratio.

Empirical formula calculation Example 5.1 The compound formed between lead and sulfur

It is found that 207g of lead combined with 32g of sulphur to form 239g of lead sulphide.

From the data work out the formula of lead sulphide. (Relative atomic masses: Pb = 207 and S = 32)

In this case it easy to see that by the atomic mass ratio, 239 splits on a 1 to 1 basis of 1 atom of lead to 1 atom of sulphur (1 x 207 to 1 x 32 by mass)

so the formula is simply PbS

You can set out the calculation in a simple table format, in this case the numbers are very easy to deal with!

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Empirical formula calculation Example 5.2 The empirical formula of a lead oxide
Empirical formula calculation Example 5.3 The empirical formula of aluminium sulfide

It is found that 54g of aluminium forms 150g of aluminium sulphide.

Work out the formula of aluminium sulphide. (Relative atomic masses: Al = 27 and S = 32).

Amount of sulphur combined with the aluminium = 150 - 54 = 96g

By atomic ratio, the 54 of aluminium is equivalent to 2 atoms of aluminium and the 96 of sulphur is equivalent to3 atoms of sulphur.

Therefore the atomic ratio is 2 to 3,

so the formula of aluminium sulphide is Al₂S₃

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Empirical formula calculation Example 5.4 From now on, questions just using the table method to work out empirical formula from more awkward numbers! In this case a compound formed between copper and chlorine.

Empirical formula calculation Example 5.5 The empirical formula of a compound of carbon and chlorine

Empirical formula calculation Example 5.6 The formula of a hydrocarbon.

Empirical formula calculation Example 5.7 The analysis of sodium sulfate, calculating its empirical formula from the % composition by mass.

Empirical formula calculation Example 5.8 The formula of a hydrocarbon.

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See section 8. for more empirical/molecular formula calculations involving moles.

Self-assessment Quizzes: type in answer QUIZ ormultiple choice QUIZ
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Above is typical periodic table used in how to deduce and work out empirical formula science-chemistry specifications in doing chemical calculations, and I've 'usually' used these values in my exemplar calculations to cover most syllabuses
OTHER CALCULATION PAGES

Empirical formula & formula mass of a compound from reacting masses (easy start, not using moles) (this page)

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Calculating Empirical Formula from reacting masses percent composition gcse chemistry Calculations gcse chemistry igcse KS4 science A level GCE AS A2 O Level practice questions exercises

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Empirical formula and formula mass from reacting masses (easy start, no moles!) 5. The EMPIRICAL FORMULA of a compound can be worked out by knowing the exact masses of the elements that combine to form a given mass of a compound.

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s-cool

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Writing Formulae and Balancing Equations | S-cool, the revision website

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Writing Formulae and Balancing Equations revision notes and tests for GCSE Chemistry.

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s-cool is a good revision website

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Earlier we spoke about everything been made up of particles, from a lump of gold to our entire body! Now we are going to give these particles a name - atoms.

Single atoms are far too small to be seen even with the most powerful microscope. However, in spite of their tiny size, we know an awful lot about atoms.

Scientists have found that every atom contains a nucleus, and a cloud of particles called electrons that rotate around the nucleus in the centre of the atom.

We have already said that atoms consist of a nucleus surrounded by a cloud of electrons whizzing around it. However, the nucleus itself is composed of two particles, neutrons and protons.

All particles are light so scientists measure their masses in atomic mass unit instead of grams.
Protons and electrons carry a charge; neutrons do not.
A magnesium atom:

The different energy levels for the electrons are called energy shells. Each shell can hold a limited number of electrons.

The first shell can hold 2 electrons.

The second shell can hold 8 electrons.

The third shell can hold a maximum of 8 electrons.

A magnesium atom holds 2 electrons in the third shell only. This is not a full shell or octet.

The magnesium atom is often written as 2,8,2 - this is known as its electronic configuration.

The above table supplies the reader with plenty of facts about individual atoms belonging to all the known elements. This table is known as the PeriodicTable.

Each element is given a symbol, a sort of shorthand version of its name, for example Sodium has the symbol Na. Next to the symbol are two numbers, the top number (the larger of the two), is known as the mass number, the lower number (smallest of the two) is the proton number.

The periodic table is arranged in order of proton number, a hydrogen atom having the smallest!

Look at the magnesium atom diagram again. It has 12 protons. This fact is used to identify an atom since it is specific to them.

For example: only a magnesium atom has 12 protons, only a sodium atom has 11 protons!

The magnesium atom has 12 electrons. The number of electrons for an atom always equals the number of protons. This means that their opposite and equal charge cancel one another out. Atoms are neutral!

The electrons in an atom have negligible mass. So the mass number only takes into account the number of protons and neutrons.

The mass no. = the no. of protons + the no. of neutrons.

Since we already know the number of protons, as given by the proton number, we can calculate the number of neutrons.

For a magnesium atom:

Mass number = 24

Proton number =12

24 = 12 + the number of neutrons

Therefore, number of neutrons in a magnesium atom = 24 - 12 = 12 neutrons.

The atoms of an element are not always the same! Although they may contain the same number of protons their neutron numbers may differ from atom to atom.
Isotopes are atoms of the same element, with the same number of protons but different numbers of neutrons.

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Earlier we spoke about everything been made up of particles, from a lump of gold to our entire body! Now we are going to give these particles a name - atoms.Single atoms are far too small to be seen even with the most powerful microscope.

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Structure of the Leaf | Plant Biology | The Fuse School

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Learn about the structure of a leaf in this video from the "Plant Biology" chapter of the Virtual School GCSE / K12 Biology. At Fuse School, teachers and animators come together to make fun & easy-to-understand videos in Chemistry, Biology, Physics, Maths & ICT. Our OER are available free of charge to anyone.

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http://www.sciencerevisionvideo.com/ A revision video showing you how to calculate reacting masses from equations

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Calculating percentage by mass and empirical formulae

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Calculating percentage by mass and empirical formulae

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Elephants and giraffes...

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Tricksy...

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Fun

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I'm Atoms (Scientific Cover of Jason Mraz's I'm Yours)

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Well an atom's made of protons, neutrons, and electrons the first two in the nucleus, the third around it it's mostly empty space, but it feels solid in any case The elements are all the different types of atoms they differ by the number of protons in the middle.

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Atoms and isotopes

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Most people recognize that atoms are the fundamental building blocks of all matter around us. An atom itself is composed of protons, neutrons and electrons. The simplest atom is the hydrogen atom because it consists of only one proton and one electron.

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Make certain you understand

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know the definition

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Worth watching

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More about moles

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How big is a mole? (Not the animal, the other one.) - Daniel Dulek

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How much? (Moles explained)

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Stoichiometry: Chemistry for Massive Creatures - Crash Course Chemistry #6

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2 The Periodic table

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The Periodic Table: Crash Course Chemistry #4

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Start here 1 The nucleus