Essential Understandings

• Define atomic, molecular and formula mass• Calculate the molecular/formula mass of a

compound• Define Avogadro’s constant• Define the mole and relate it to mass and

molecular mass• Calculating empirical formula• Calculating molecular formulae• Finding the percentage composition

Pre-test

• For questions 1 to 5 select from the following:

• Cl Cl2 Cl- HCl CCl4 H+ • 1. A molecule of chlorine ……………

• 2. An atom of Chlorine ……………

• 3. An ion of Chlorine …………

• 4. A cation ……………

• 5. An anion ……………

• 6. Name an element (other than chlorine) whose molecules consist of two atoms. ………..

• 7. How many atoms are there in one molecule of sugar C12H22O11?• 8. How many different atoms are there in one molecule of sugar?

• 9. What is an isotope?

Atoms, Atomic Masses and Moles

• Relative atomic Masses:• Chemists use a relative atomic mass scale to compare

different atoms. In the old days they chose Hydrogen as the standard because it was the smallest atom, but when they were able to find more accurate results and when scientists recognized that elements often consisted of more than one isotope they decided to settle on one isotope. They chose one which was fairly easy to obtain and one which was a solid, so they chose 12C and they assigned this an atom mass of 12.00000g in this manner none of the previously assigned atomic masses on the old standard of hydrogen had to change too much. They just became more precise.

The Avogadro constant, moles and molar masses

• Since one atom of carbon is 12 times as heavy as one atom of hydrogen, it follows that 12g of carbon contain the same number of atoms as 1g of hydrogen. In also follows that the relative atom mass in grams of all the elements (we obtain this from the data in the Periodic Table) will contain the same number of atoms. Experiments show that this number is 6.0 X 1023 or 600,000,000,000,000,000,000,000. This number is called the Avogadro constant

• As relative atomic mass in grams of all elements contain 6.0 X 1023 atoms, chemists call 6.0 X 1023 atoms of an element 1 mole of atoms. (Symbol mol)

• Strictly speaking, the mole is defined as the amount of substance which contains the same number of particles as there are in 12.0000g of 12C

• Relative Atomic Mass Ar is the mass of an atom of an element compared to 12C

• When we move from elements to compounds then we use Relative Molar Mass Mr

• Now we come to the first of the important mole equations you will need;

Moles = n = Number of particles 6.0 X 1023 or

Moles = n = ____ # _____ 6.0 X 1023

Questions

10. How many moles are there in 1 million atoms of Ge (1 X 106)

11. How old are you? (remember IB requires units)12. How many seconds have you been on this

Earth?13. How many moles is this?14. If the Earth is 4.5 billion years old, how many

seconds is this?15. What fraction of a mole is this?

• So, you can see we are dealing in mind boggling numbers so chemists generally just talk about moles or fractions thereof, but we understand that each mole contains 6.0 X 1023 number of particles.

• The next important mole equation is to relate moles to mass in grams we use:

Moles = n = Mass in grams Relative Molar Mass or Moles = n = __m__ Mr

So what have moles to do with the Sahara desert?

16. Make a guess of how many moles of sand grains there are in the Sahara desert.1,000,000 100,000 10,000 1,000 100 10 1 0.1

17. If the Sahara desert is 4,500km from East to West, 1500km north to South and the sand is 10m deep and 1cm3 of sand contains 800 grains. How many grains of sand are there in the Sahara desert?

18. How many moles is this?

19. What is the number of moles in 10.0g of 12C?20. What is the mass in grams of 2.5 moles of 24Mg?21. What is the mass of 0.00125 moles of 31P ?

So dealing with elements is relatively easy. What about diatomic molecules such as Chlorine Cl2?

Here we use 2 X 35.5 for Mr and plug into the previous equation.

22. How many moles are there in 2.45g of chlorine molecules?23. What is the mass of 0.55 moles of Bromine liquid (Br2) if Ar of Bromine

is 80g mol-1

24. How many moles of oxygen molecules are there in 1.00g of O2 ?

25. Notice this is different from how many atoms of oxygen are there in 1.00g of O2 because there are …….. atoms of Oxygen in one …………. of oxygen.

• Now let us go one step further and move to compounds. The Mr for compounds is found by adding up the Ar of all the elements in the compound. For example CuO is 64 for copper + 16 for oxygen making a total of 80g mol-1

25. What is the Mr of NaOH?

26. How many moles are there in 10g of NaOH?27. What is the Mr of CuCO3 ? (Cu + C + O + O +O)

28. What is the mass of 0.245mol of CuCO3 ?

29. The next step is to try more complicated formulae. For example what is the Mr of FeSO4.7H2O ? This will be

Fe + S + O + O + O + O + 7( H + H + O)30. What is Mr of CuSO4.5H20 ?

31. How much hydrated copper(II)sulphate (the stuff in above question) would you need to weigh out if you needed 0.20 mole ?

32. What is Mr K2Cr2O7 ?

33. How many moles of potassium dichromate are there in 1.00 g of K2Cr2O7 ?

Sometimes the chemical formulae are displayed showing the bonds but you can easily add up all the atoms to find Mr. For example Methyl orange

H3C O II

N----C6H4----N=N----C6H4----S----O----H II H3C O• Can be reduced to N3C14H15SO3

Percentage composition• Sometimes it is important to know a percentage

composition. For example a farmer needs a nitrogen based fertilizer and he wants to get as much nitrogen as possible for his money.

• Let’s say we have a choice of 3 nitrogen based fertilizers, ammonium sulphate, (NH4)2SO4, ammonium nitrate, NH4NO3 and Urea , CO(NH2)2 and each costs 150 bombles(the Universal currency) a kilo. Which should he buy?

• One way of looking at it is to consider the amount of nitrogen as a fraction of the whole and then multiply by 100 to obtain a %

For ammonium sulphate there are 2 Nitrogen atoms N2 X 100 = 28 X 100 = 21.2%

(NH4)2SO4 132

For ammonium nitrate there are also 2 Nitrogen atoms in the formula

N2 X 100 = 28 X 100 = 35%

NH4NO3 80

33. Please complete the calculation for Urea and state which fertilizer the farmer should buy.

• Empirical formula• This is the simplest whole number ratio of atoms in a chemical

formula. So if the real formula (called the molecular formula if it is a molecule) of glucose is C6H12O6 then the empirical formula of glucose, which is a carbohydrate is CH2O since it can be simplified by dividing through by 6. CH2O is the empirical formula for a whole range of carbohydrates.

• Empirical formula, like most chemical formulae, are found by experiment.

• Suppose an experiment on ethene, a hydrocarbon (containing C and H only) found that it contained 85.72% carbon and 14.28% hydrogen, then here is how we find the empirical formula.

• Remember that all formula are in whole number mole ratios, so the first thing to do is to convert each mass into moles. A second good idea is to remember this template for all empirical formula calculations.

Remember this template

C H

Mass in grams 85.72 14.28

Moles 85.72/12 14.28/1

Ratio 7.14 14.28

Simple ratio 1 2

Empirical formula

CH2

What to do if you have horrible numbers• If you have ‘horrible’ numbers in the ratio line just

divide both sides by the smallest ‘horrible’ number then you will have 1: something.

• If the ratio is 1 : something which is not near a whole number, then you have to keep multiplying each side by 2, then 3 then 4 etc until both sides become whole numbers, but all columns must be X same number

• Often the experimental results are not given as a percentage and may look something like this:

• A 1.31g sample of sulphur was allowed to react with an excess of chlorine to produce 4.22g of a product that contains only sulphur and chlorine. What is it’s empirical formula?

S Cl

Mass/g

1.31 4.22-1.31

Moles 1.31/32 2.91/35.5

Ratio 0.04094 0.08197

Simple ratio

1 2

Empirical formula

SCl2

• Besides there being ‘horrible’ ratios there are often more than two elements in a compound.

• Vanillin, has the following composition by mass carbon, 63.2%; hydrogen 5.26% and the remainder is oxygen. What is the empirical formula of vanillin?

C H O

Mass 63.2 5.26 100 -(63.2+5.26)

Moles 63.2/12 5.26/1 31.54/16

Ratio 5.2666 5.26 1.9713

Ratio 2.67 2.67 1

Simple

Ratio8 8 3

Empirical formula

C8H8O3

34. A propellant used in an aerosol can was analysed and found to contain 0.423g of carbon, 2.50g of chlorine, and 1.34g of fluorine. What is the empirical formula of this compound?

C Cl F

Mass/g 0.423 2.50 1.34

Moles

Ratio

Ratio

Simple Ratio

Empirical

• Going from an empirical formula to a Molecular formula• Not only does the molecular formula give us the ratio of atoms in

the compound, it also tells us the actual number of atoms of each kind in a molecule of the substance.

• In our previous example with glucose, the empirical formula is CH2O so how do we get to the molecular formula of C6H12O6 ? We know there is a whole range of carbohydrates so how can we distinguish which carbohydrate? One important way in which these substances differ is that they have different molecular masses. For instance the molecular mass of glucose is 180g mol-1 and this must be a whole number multiple of the mass of CH2O In other words we find out how many units of CH2O add up to make a mass of 180, the mass of 1 mole in the following manner:

• x(CH2O) = 180

• x(12+2+16) = 180• 30x = 180• x = 6 • (There are 6 units of CH2O in glucose)• The molecular formula is written as C6H12O6

35. If the empirical formula of a compound was found to be NaS2O3 and it’s Formula Mass (the name we use for ionic compounds instead of molecular mass for covalent molecules) is 270.4. What is the correct formula?

This looks pretty Complicated so let us just

Consider Room Temperature and

Pressure, then 1moleOf any gas occupies24 liters =24,000cm3

You can thank Mr Avogadro for this

Let us now consider chemical equations