amount of substances

Question 1

what is the relative atomic mass Ar

Answer 1

masses are compared and the relative masses are used as the actual mass in grams of any atom or molecule is too tiny to find by weighing

the actual definition is:
the relative atomic mass A, is the weighted average mass of an atom of an element, taking into account its naturally occurring isotopes, relative to 1/12 the relative atomic mass of an atom of carbon -12

OR

the average mass of one atom of an element/ (1/12) mass of one atom of C12

Question 2

what is the relative molecular mass

Answer 2

the relative molecular mass Mr = average mass of one molecule/ (1/12) mass of one atom of C12

Question 3

what is Avogadro constant and the mole

Answer 3

one atom of any element is too small to see with an optical microscope and impossible to weigh individually

so, to count atoms, chemists must weigh large numbers of them

the Avogadro constant or Avogadro number is the number amount of substance in grams that has the same number of particles as there are atoms in 12 grams of carbon 12

OR

6.022 x 10(23)

Question 4

what is the mole

Answer 4

the amount of substance that contains 6.022 x 10(23) particles is called a mole

the relative mass of any element in grams contains one mole of atoms

Question 5

how can you work out the number of moles

Answer 5

you can use this equation:

number of moles = mass/mr

to work out the number of moles

Question 6

what is a solution

Answer 6

a solution consists of a solvent and a solute dissolved in it

Question 7

what is the concentration of a solution

Answer 7

the concentration of a solution tells us how much solute is present i a known volume of solution

Question 8

what is the unit of concentration

Answer 8

the concentration of a solution are measured in mol dm-3

1 mol dm-3 means there is 1 mole of solute per cubic decimetre of solution

Question 9

how do we work out the concentration of a solution

Answer 9

e.g workeed example
finding the concentration in mol dm-3

1.17g of sodium chloride was dissolved in water to make 500cm3 of solution
What is the concentration of the solution in mol dm-3

1st work out the moles

Na = 23.0 Cl= 35.5

number of moles= mass (g)/ mr

the mr of NaCL is 58.5
the number of moles = 1.17/ 58.5
=0.02

this is dissolved in 500cm3
1000cm = 1dm3 therefore its is 0.5dm3

we can then use this equation: concentration= number of moles/volume

=0.04 mol dm-3

Question 10

what is the equation that is used to work out the concentration of a solution

Answer 10

concentration = number of moles/ volume

Question 11

what equation do we use to work out the number of moles in a solution

Answer 11

number of moles = concentration x volume (cm3)/1000

Question 12

what is boyle’s law

Answer 12

the product of pressure and volume is a constant ass long as the temperature remains constant:

pressure P x volume V = constant

Question 13

what is charles’s law

Answer 13

the volume is proportional to the temperature as long as the pressure remains constant

volume V ∝ temperature T

and volume/ temperature = constant

Question 14

what is Gay - lussac’s law (also called the volume law)

Answer 14

the pressure is proportional to the temperature as long as the volume remains constant

Question 15

what happens when we combine all the laws together / what is the ideal gas equation

Answer 15

combining these relationships gives us the equation

=PV =nRT

P = pressure (Pa) 
V = volume = (m3) 
n= number of moles 
R= gas constant 
T = temperature (C+ 273)  

the value R is always 8.31

Question 16

why do no gases hold the ideal gas equation perfectly

Answer 16

the volume of a given mass of any gas is not fixed. It changes with pressure and temperature.

Question 17

a worked example of using the ideal equation

Answer 17

worked example: Finding the number of mols

How many moles of hydrogen molecules are present in a volume of 100cm3 at a temperature of 20.0C and a pressure of 100kPa

1st P must be in Pa, and 100kPa = 100 00 ( x1000)
V must be in m3, and 100cm3 = 100 x 10-6

t = 293k

n= 100000 x 100x 10-6/ constant

=0.00411 moles

Question 18

what is the atom economy

Answer 18

once you balanced a chemical reaction, you can calculate the theoretical amount that you should be able to make of any of the products

we determine the atom economy as an atom economy

Question 19

how do we calculate the atom economy

Answer 19

mr of desired product / total mr of reactants x 100

Question 20

what is the difference between the atom economy and the percentage yield

Answer 20

the yield of a reaction is different from the atom economy

the atom economy tells us in theory how many atoms must be wasted in a reaction

the yield tells us about the practical efficiency of the process, how much is lost

Question 21

what is the equation for percentage yield

Answer 21

the yield of a chemical reaction =

mass of product actually made/ maximum theoretical mass product x 100

Question 22

why are titrations important

Answer 22

you can do a titration to find the concentration of an acid or an alkali

But first you will have to prepare a standard solution to use in your titration

Question 23

what is a standard solution

Answer 23

it is a solution that you know the exact concentration of

making a standard solution involves dissolving a known amount of solid in a known amount of water to create a known concentration

Question 24

how do we create a standard solution

Answer 24

e. g. make 250cm3 of a 2.00 mol dm3 solution of sodium
1. first work out how many mols of sodium hydroxide you need using the formula

moles = concentration x volume
= 2 x 0.250 = 0.500

2. now work out how many grams of sodium hydroxide you need using the formula mass = moles x Mr

= 20.0g

3. place a weighing bottle on a digital balance. Weigh out the required mass of solid approximately and tip into a beaker
4. weigh the weighing bottle ( which may stil contain traces of the solid) subtract the mass of the bottle from the mass of the bottle and the solid together to find the precise mass of the solid you have weighed out ( weigh by difference)
5. add distilled water to the beaker and stir until all the sodium hydroxide is dissolved
6. tip the sodium into a 250cm3 volumetric flask - use a funnel to make sure it all goes in
7. rinse the beaker, stirring rod and funnel with distilled water and add that to the flask too. This makes sure there’s no solute clinging to the beaker or rod
8. now top the flask up to the correct volume wit more distilled water. Makes sure the bottom meniscus reaches the line. When you get to the line add water drop by drop - if you go over the line you have to start again
9. stopper the flask and turn it upside down a few times to make sure its mixed well
10. now calculate the exact concentration of your standard solution

Question 25

how do you carry out a titration

Answer 25

1. start off by using a pipette to measure out a set volume of the solution that you want to know the concentration of
2. put it in a flask
3. add a few drops of an appropriate indicator to the flask
4. then fill a burette with a standard solution of the acid- in a standard solution, you know the concentration
5. use a funnel to carefully pour the acid into the burette - always do this below eye level to avoid any acid splashing on to your face or eyes
6. first do a rough titration to get an idea point where the alkali is neutralised and the indicator changes colour (end point)
7. take an initial reading of how much acid is in the burette - then gradually add the acid to the alkali, giving the flask a regular swirl
8. when the colour changes, take a final reading
9. then do an accurate titration
10. take an initial reading, then run the acid to within 2cm3 of the end point
    - when you get to this stage, add it drop wise
    - if you don’t see exactly when the colour changes you’ll overshoot and your result won’t accurate
11. find the amount of acid used to neutralise the alkali by subtracting the final reading from the initial reading. This is called the titre
12. repeat the titration a few times, until you have at least three concordant results ( within 0.1cm3 of each other)
13. use the results from each repeat to calculate the mean volume acid used.
    - remember to leave out any anomalous results when calculating your mean - anonmalous results can distort your answer

Question 26

what are indicators

Answer 26

indicators change colour quickly over a very small pH change

• they are used so you know the reaction has ended

Question 27

what are the 2 indicators

Answer 27

methyl orange:
yellow in alkali, red in acids

phenolphalein
colourless in acids,
pink in alkalis

it’s a good idea to stand your flask on a white tile when you’re titrating - it’ll make it easier to see exactly when the colour changes